As filed with the Securities and Exchange Commission on May 9, 2023

No. 333-          

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-3

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

Integral Ad Science Holding Corp.

(Exact name of registrant as specified in its charter)

Not Applicable¹

(646) 278-4871

(Address, including zip code, and telephone number, including area code, of registrant’s principal executive offices)

Lisa Utzschneider

Chief Executive Officer

c/o The Corporation Trust Company

Corporation Trust Center

1209 Orange Street

Wilmington, Delaware 19801

(302) 777-0200

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copies to:

Robert M. Hayward, P.C.

Robert E. Goedert, P.C.

Michael P. Keeley, P.C.

Kirkland & Ellis LLP

300 North LaSalle

Chicago, IL 60654

(312) 862-2000

Approximate date of commencement of proposed sale to the public: From time to time after this Registration Statement becomes effective.

If the only securities being registered on this Form are being offered pursuant to dividend or interest reinvestment plans, please check the following box.  ☐

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, other than securities offered only in connection with dividend or interest reinvestment plans, check the following box.  ☒

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

If this Form is a registration statement pursuant to General Instruction I.D. or a post-effective amendment thereto that shall become effective upon filing with the Commission pursuant to Rule 462(e) under the Securities Act, check the following box.  ☒

If this Form is a post-effective amendment to a registration statement filed pursuant to General Instruction I.D. filed to register additional securities or additional classes of securities pursuant to Rule 413(b) under the Securities Act, check the following box.  ☐

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act.  ☐

PROSPECTUS

Integral Ad Science Holding Corp.

Common Stock

This prospectus covers the resale by the selling stockholders, to be identified in one or more supplements to this prospectus, from time to time in amounts, at prices, and on terms that will be determined at the time of any such offering, of shares of common stock, par value $0.001 per share (“common stock”), of Integral Ad Science Holding Corp. (the “Company,” “we” or “us”).

We are not selling any shares of common stock under this prospectus, and we will not receive any of the proceeds from the sale or other disposition of shares of common stock by the selling stockholders.

These securities may be offered and sold by the selling stockholders from time to time in accordance with the provisions set forth under “Plan of Distribution.” The selling stockholders may offer and sell these securities to or through one or more underwriters, dealers or agents, who may receive compensation in the form of discounts, concessions or commissions, or directly to purchasers, on a continuous or delayed basis. The selling stockholders may offer and sell these securities at various times in amounts, at prices and on terms to be determined by market conditions and other factors at the time of such offerings. This prospectus describes the general terms of these securities and the general manner in which the selling stockholders will offer and sell these securities. A prospectus supplement or free writing prospectus, if needed, will identify the selling stockholders, describe the specific manner in which the selling stockholders will offer and sell these securities and also may add, update, or change information contained or incorporated by reference in this prospectus. The names of any underwriters and the specific terms of a plan of distribution, if needed, will be stated in a prospectus supplement or a free writing prospectus.

Our common stock is quoted on the NASDAQ Global Select Market (“NASDAQ”) under the symbol “IAS.” The last reported closing sale price of our common stock on NASDAQ was $16.96 per share on May 8, 2023.

The Company is an “emerging growth company” as the term is used in the Jumpstart Our Business Startups Act of 2012 and, as such, has elected to comply with certain reduced public company reporting requirements for this prospectus and expects to continue to do so in future filings.

INVESTING IN OUR COMMON STOCK INVOLVES A NUMBER OF RISKS. SEE “RISK FACTORS” ON PAGE 6 OF THIS PROSPECTUS AND IN THE APPLICABLE PROSPECTUS SUPPLEMENT AND THE RISK FACTORS IN THE DOCUMENTS INCORPORATED BY REFERENCE IN THIS PROSPECTUS AND IN THE APPLICABLE PROSPECTUS SUPPLEMENT TO READ ABOUT FACTORS YOU SHOULD CAREFULLY CONSIDER BEFORE INVESTING IN OUR COMMON STOCK.

Neither the Securities and Exchange Commission (the “SEC”) nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

The date of this prospectus is May 9, 2023.

TABLE OF CONTENTS

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ABOUT THIS PROSPECTUS

This prospectus is part of a registration statement that we have filed with the SEC using a “shelf” registration process as a “well-known seasoned issuer” as defined in Rule 405 under the Securities Act of 1933, as amended (the “Securities Act”). Using this process, the selling stockholders may offer the securities described in this prospectus in one or more offerings. This prospectus provides you with a general description of us and the securities that may be offered by the selling stockholders. Because each of the selling stockholders may be deemed to be an “underwriter” within the meaning of the Securities Act, each time securities are offered by the selling stockholders pursuant to this prospectus, the selling stockholders may be required to provide you with this prospectus and, in certain cases, a prospectus supplement that will contain specific information about the selling stockholders and the terms of the securities being offered. We may also provide you with a free writing prospectus that contains information about the specific terms of that offering, including the name of any selling stockholder and the prices at which the shares of our common stock will be sold. Any prospectus supplement and any free writing prospectus may also add to, update, or change the information contained in this prospectus. If there is any inconsistency between the information in this prospectus and any prospectus supplement or free writing prospectus, you should rely on the information in such prospectus supplement or free writing prospectus. Please carefully read this prospectus and the applicable prospectus supplement or free writing prospectus, in addition to the information contained in the documents we refer to under the headings “Where You Can Find More Information” and “Information Incorporated by Reference.”

Neither we nor the selling stockholders nor any underwriter have authorized anyone to provide you with any information or to make any representations other than those contained or incorporated by reference in this prospectus, the applicable prospectus supplement or free writing prospectus made available by us. Neither we, the selling stockholders nor any underwriter take any responsibility for, and can provide no assurance as to the reliability of, any other information that others may give you.

You should assume that the information appearing in this prospectus is accurate only as of the date on the cover of this prospectus and that any information we have incorporated by reference is accurate only as of the date of the document incorporated by reference. Our business, financial condition, results of operations and prospects may have changed since the date indicated on the cover page of such documents.

The distribution of this prospectus may be restricted by law in certain jurisdictions. You should inform yourself about and observe these restrictions. This prospectus does not constitute, and may not be used in connection with, an offer or solicitation by anyone in any jurisdiction in which the offer or solicitation is not authorized, or in which the person making the offer or solicitation is not qualified to do so, or to any person to whom it is unlawful to make the offer or solicitation.

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RISK FACTORS

Before making an investment decision, you should carefully consider the risks described under “Risk Factors” in our most recent Annual Report on Form 10-K and in subsequently filed Quarterly Reports on Form 10-Q, together with all of the other information appearing in this prospectus or the applicable prospectus supplement or incorporated by reference herein or therein. The risks so described are not the only risks facing our company. Additional risks not presently known to us or that we currently deem immaterial may also impair our business operations. Our business, financial condition and results of operations could be materially adversely affected by any of these risks. Furthermore, the trading price of our common stock could decline due to any of these risks, and you may lose all or part of your investment.

Risks Related to our common stock and any offering of our common stock

Vista controls us, and their interests may conflict with ours or yours in the future.

Investment entities affiliated with Vista control a majority of the voting power of our outstanding common stock, which means that, based on its percentage voting power, Vista controls the vote of all matters submitted to a vote of our stockholders. This control enables Vista to control the election of the members of our board of directors (the “Board”) and all other corporate decisions. Even when Vista ceases to control a majority of the total voting power, for so long as Vista continues to own a significant percentage of our common stock, Vista will still be able to significantly influence the composition of our Board and the approval of actions requiring stockholder approval. Accordingly, for such period of time, Vista will have significant influence with respect to our management, business plans and policies, including the appointment and removal of our officers, decisions on whether to raise future capital and amending our Certificate of Incorporation and Bylaws, which govern the rights attached to our common stock. In particular, for so long as Vista continues to own a significant percentage of our common stock, Vista will be able to cause or prevent a change of control of us or a change in the composition of our Board and could preclude any unsolicited acquisition of us. The concentration of ownership could deprive you of an opportunity to receive a premium for your shares of common stock as part of a sale of us and ultimately might affect the market price of our common stock.

The market price of our common stock may be volatile and could decline.

Volatility in the market price of our common stock may prevent you from being able to sell your shares at or above the price you paid for your shares. The market price of our common stock may fluctuate significantly. Among the factors that could affect our stock price are:

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We cannot assure you that you will be able to resell your shares at or above the price you paid for your shares. Stock markets have experienced extreme volatility in recent years that has been unrelated to the operating performance of particular companies. These broad market fluctuations may adversely affect the trading price of our common stock. In the past, following periods of volatility in the market price of a company’s securities, class action litigation has often been instituted against the affected company. Any litigation of this type brought against us could result in substantial costs and a diversion of our management’s attention and resources, which could materially and adversely affect our business, financial position, results of operations, and cash flows.

An active, liquid trading market for our common stock may not be sustained.

Although our common stock is currently quoted on NASDAQ under the symbol “IAS,” an active trading market for our shares may not be sustained. Accordingly, if an active trading market for our common stock is not maintained, the liquidity of our common stock, your ability to sell your shares of our common stock when desired, and the prices that you may obtain for your shares of common stock will be adversely affected.

Future sales of shares by us or our existing stockholders could cause our stock price to decline.

Sales of substantial amounts of our common stock in the public market, or the perception that such sales could occur, could cause the market price of our common stock to decline. Such sales, or the possibility that such sales may occur, also might make it more difficult for us to sell equity securities in the future at a time and at a price that we deem appropriate.

All of the common stock shares to be sold in any offering will be immediately tradable without restriction under the Securities Act, except for any shares held by “affiliates,” as that term is defined in Rule 144 under the Securities Act. In connection with such a sale, shares held by Vista (other than those contemplated to be sold in such offering), our executives and our directors may be subject to a customary lock-up agreement to be entered into by us, the selling stockholders, and our executive officers and directors.

We are also party to a registration rights agreement, dated as of July 2, 2021, with the Vista Funds and certain other investors, pursuant to which the shares of our common stock and our other equity securities held by them will be eligible for resale, subject to certain limitations set forth therein.

On July 1, 2021, we also filed a registration statement on Form S-8 under the Securities Act to register the shares of common stock to be issued under our 2021 Omnibus Incentive Plan. As a result, all shares of common stock acquired upon the vesting of restricted stock units, market stock units and other securities convertible or exchangeable into shares of common stock granted under our 2021 Omnibus Incentive Plan will be freely tradable under the Securities Act, subject to the terms of the lock-up agreements, unless purchased by our affiliates.

Additionally, in connection with any offering, the selling stockholders may enter into lock-up agreements under which they may agree not to, among other things and subject to certain exceptions, offer, sell, contract to sell, pledge, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase, engage in any hedging or similar transaction or arrangement, lend or otherwise transfer or dispose of, directly or indirectly, any of our securities that are substantially similar to the securities offered thereby, without the prior written consent of the underwriters, for a period to be determined at the time of such offering. Any such lock-up agreements may provide that the underwriters can waive such restrictions in their sole discretion at any time.

Because we have no current plans to pay regular cash dividends on our common stock, you may not receive any return on investment unless you sell your common stock for a price greater than that which you paid for it.

We do not anticipate paying any regular cash dividends on our common stock. Any decision to declare and pay dividends in the future will be made at the discretion of our Board and will depend on, among other things,

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our results of operations, financial condition, cash requirements, contractual restrictions, and other factors that our Board may deem relevant. In addition, our ability to pay dividends is, and may be, limited by covenants of existing and any future outstanding indebtedness we or our subsidiaries incur. Therefore, any return on investment in our common stock is solely dependent upon the appreciation of the price of our common stock on the open market, which may not occur.

We are a “controlled company” within the meaning of the rules of NASDAQ and, as a result, we qualify for, and have relied upon, exemptions from certain corporate governance requirements. Therefore, you do not have the same protections as those afforded to shareholders of companies that are subject to such governance requirements.

Vista controls a majority of the voting power of our outstanding common stock. As a result, we are a “controlled company” within the meaning of the corporate governance standards of NASDAQ. Under these rules, a company of which more than 50% of the voting power for the election of directors is held by an individual, group or another company is a “controlled company” and may elect not to comply with certain corporate governance requirements, including:

We have utilized and expect to continue utilizing these exemptions as long as they are available to us. As a result, we may not have a majority of independent directors on our Board and our compensation and nominating committee may not consist entirely of independent directors. Accordingly, you do not have the same protections afforded to shareholders of companies that are subject to all of the corporate governance requirements of the rules of NASDAQ.

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FORWARD-LOOKING STATEMENTS

Certain statements in or incorporated by reference in this prospectus and any prospectus supplement are “forward-looking statements” within the meaning of the Private Securities Litigation Reform Act of 1995, Section 27A of the Securities Act and Section 21E of the Exchange Act that are subject to risks and uncertainties. All statements other than statements of historical fact included in this prospectus and any prospectus supplement are forward-looking statements (including statements herein and found in documents incorporated herein by reference, to the extent applicable). Forward-looking statements give our current expectations and projections relating to our financial condition, results of operations, plans, objectives, future performance and business. You can identify forward-looking statements by the fact that they do not relate strictly to historical or current facts. These statements may include words such as “anticipate,” “estimate,” “expect,” “project,” “plan,” “intend,” “believe,” “may,” “will,” “should,” “can have,” “likely” and other words and terms of similar meaning in connection with any discussion of the timing or nature of future operating or financial performance or other events. For example, all statements we make relating to our estimated and projected costs, expenditures, cash flows, growth rates and financial results or our plans and objectives for future operations, growth initiatives, or strategies are forward-looking statements. All forward-looking statements are subject to risks and uncertainties that may cause actual results to differ materially from those that we expected, including those contained under “Risk Factors” in our Annual Report on Form 10-K for the year ended December 31, 2022, which is incorporated in this prospectus by reference (and in any of our Annual Reports on Form 10-K and Quarterly Reports on Form 10-Q for subsequent periods, in our other filings with the SEC, and any risks identified under “Risk Factors” in this prospectus and any accompanying prospectus supplement).

We derive many of our forward-looking statements from our operating budgets and forecasts, which are based on many detailed assumptions. While we believe that our assumptions are reasonable, we caution that it is very difficult to predict the impact of known factors, and it is impossible for us to anticipate all factors that could affect our actual results. Important factors that could cause actual results to differ materially from our expectations, or cautionary statements, are disclosed under “Risk Factors” in our Annual Report on Form 10-K for the year ended December 31, 2022 and “Management’s Discussion and Analysis of Financial Condition and Results of Operations” in our Annual Report on Form 10-K for the year ended December 31, 2022 and Quarterly Report on Form 10-Q for the Quarter Ended March 31, 2023, each of which are incorporated in this prospectus by reference (and in any of our Annual Reports on Form 10-K and Quarterly Reports on Form 10-Q for subsequent periods and in our other filings with the SEC). All written and oral forward-looking statements attributable to us, or persons acting on our behalf, are expressly qualified in their entirety by these cautionary statements as well as other cautionary statements that are made from time to time in our other SEC filings and public communications. You should evaluate all forward-looking statements made in the context of these risks and uncertainties.

We caution you that the important factors referenced above may not contain all of the factors that are important to you. In addition, we cannot assure you that we will realize the results or developments we expect or anticipate or, even if substantially realized, that they will result in the consequences or affect us or our operations in the way we expect. The forward-looking statements included in this prospectus and any prospectus supplement, or any document incorporated by reference, are made only as of the date hereof or thereof (as applicable). We undertake no obligation to update or revise any forward-looking statement as a result of new information, future events or otherwise, except as otherwise required by law.

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USE OF PROCEEDS

We are not selling any shares of common stock under this prospectus, and we will not receive any of the proceeds from the sale of shares of common stock by the selling stockholders. All shares of common stock offered from time to time pursuant to this prospectus are being registered for the account of the selling stockholders.

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SELLING STOCKHOLDERS

Information regarding the identities of any selling stockholders, any material relationships the selling stockholders have had with the Company within the past three years, the beneficial ownership of our common stock by the selling stockholders, the number of shares being offered by such selling stockholders and the number of shares beneficially owned by the selling stockholders after the applicable offering will be set forth in a prospectus supplement, free writing prospectus, post-effective amendment, or filings we make with the SEC under the Securities Exchange Act of 1934, as amended (the “Exchange Act”) that are incorporated by reference. No selling stockholder may sell any shares of our common stock pursuant to this prospectus until we have identified such selling stockholder and the shares being offered for resale by such selling stockholder in a subsequent prospectus supplement, free writing prospectus, or in a post-effective amendment. However, selling stockholders may sell or transfer all or a portion of their shares of our common stock pursuant to any available exemption from the registration requirements of the Securities Act.

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DESCRIPTION OF COMMON STOCK

The description of our common stock is incorporated by reference to Exhibit 4.1 to our Annual Report on Form 10-K for the year ended December 31, 2021, filed with the SEC on March 2, 2022. For more information regarding our common stock, see our Certificate of Incorporation and Bylaws incorporated herein by reference and filed as Exhibits 3.1 and 3.2 hereto, respectively.

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DIVIDEND POLICY

We have never declared or paid any cash dividends on our common stock. We currently intend to retain any future earnings and do not expect to pay any dividends in the foreseeable future. Additionally, because we are a holding company, our ability to pay dividends on our common stock may be limited by restrictions on the ability of our subsidiaries to pay dividends or make distributions to us. Any future determination to declare cash dividends will be made at the discretion of our board of directors, subject to applicable laws, and will depend on a number of factors, including our financial condition, results of operations, capital requirements, contractual restrictions, general business conditions, and other factors that our board of directors may deem relevant.

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MATERIAL U.S. FEDERAL INCOME TAX CONSEQUENCES TO NON-U.S. HOLDERS

The following discussion is a summary of the material U.S. federal income tax consequences to Non-U.S. Holders (as defined below) of the purchase, ownership and disposition of our common stock sold pursuant to this prospectus, but does not purport to be a complete analysis of all potential tax effects. The effects of other U.S. federal tax laws, such as estate and gift tax laws, and any applicable state, local or non-U.S. tax laws are not discussed. This discussion is based on the Internal Revenue Code of 1986, as amended, or the Code, Treasury regulations promulgated thereunder, or Treasury Regulations, judicial decisions, and published rulings and administrative pronouncements of the U.S. Internal Revenue Service, or the IRS, in each case as in effect as of the date hereof. These authorities may change or be subject to differing interpretations. Any such change or differing interpretation may be applied retroactively in a manner that could adversely affect a Non-U.S. Holder of our common stock. We have not sought and will not seek any rulings from the IRS regarding the matters discussed below. There can be no assurance the IRS or a court will not take a contrary position to those discussed below regarding the tax consequences of the purchase, ownership, and disposition of our common stock.

This discussion is limited to Non-U.S. Holders that hold our common stock as a “capital asset” within the meaning of Section 1221 of the Code (generally, property held for investment). This discussion does not address all U.S. federal income tax consequences relevant to a Non-U.S. Holder’s particular circumstances, including the impact of the Medicare contribution tax on net investment income. In addition, it does not address consequences relevant to persons subject to special rules, including, without limitation:

If any partnership or other entity or arrangement classified as a partnership for U.S. federal income tax purposes holds our common stock, the tax treatment of a partner in the partnership will depend on the status of the partner, the activities of the partnership, and certain determinations made at the partner level. Accordingly, partnerships holding our common stock and partners in such partnerships should consult their tax advisors regarding the U.S. federal income tax consequences to them.

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THIS DISCUSSION IS FOR INFORMATIONAL PURPOSES ONLY AND IS NOT TAX ADVICE. INVESTORS SHOULD CONSULT THEIR TAX ADVISORS WITH RESPECT TO THE APPLICATION OF THE U.S. FEDERAL INCOME TAX LAWS TO THEIR PARTICULAR SITUATIONS AS WELL AS ANY TAX CONSEQUENCES OF THE PURCHASE, OWNERSHIP, AND DISPOSITION OF OUR COMMON STOCK ARISING UNDER THE U.S. FEDERAL ESTATE OR GIFT TAX LAWS OR UNDER THE LAWS OF ANY STATE, LOCAL, OR NON-U.S. TAXING JURISDICTION OR UNDER ANY APPLICABLE INCOME TAX TREATY.

Definition of a Non-U.S. Holder

For purposes of this discussion, a “Non-U.S. Holder” is any beneficial owner of our common stock that is neither a “United States person” nor a partnership or other entity or arrangement treated as a partnership for U.S. federal income tax purposes. A United States person is any person that, for U.S. federal income tax purposes, is or is treated as any of the following:

Distributions

As described in the section entitled “Dividend Policy,” we do not anticipate declaring or paying dividends to holders of our common stock in the foreseeable future. However, if we make distributions of cash or property on our common stock, such distributions will constitute dividends for U.S. federal income tax purposes to the extent paid from our current or accumulated earnings and profits, as determined under U.S. federal income tax principles. Amounts not treated as dividends for U.S. federal income tax purposes will constitute a non-taxable return of capital and first be applied against and reduce a Non-U.S. Holder’s adjusted tax basis in its common stock, but not below zero. Any excess will be treated as capital gain and will be treated as described below under “Sale or Other Taxable Disposition.”

Subject to the discussion below on effectively connected income, backup withholding, and the Foreign Account Tax Compliance Act, dividends paid to a Non-U.S. Holder of our common stock will be subject to U.S. federal withholding tax at a rate of 30% of the gross amount of the dividends (or such lower rate specified by an applicable income tax treaty, provided the Non-U.S. Holder furnishes to the applicable withholding agent prior to the payment of dividends a valid IRS Form W-8BEN or W-8BEN-E (or other applicable documentation) certifying qualification for the lower treaty rate). A Non-U.S. Holder that does not timely furnish the required documentation, but that qualifies for a reduced treaty rate, may obtain a refund of any excess amounts withheld by timely filing an appropriate claim for refund with the IRS. Non-U.S. Holders should consult their tax advisors regarding their entitlement to benefits under any applicable income tax treaty.

If dividends paid to a Non-U.S. Holder are effectively connected with the Non-U.S. Holder’s conduct of a trade or business within the U.S. (and, if required by an applicable income tax treaty, the Non-U.S. Holder maintains a permanent establishment in the U.S. to which such dividends are attributable), the Non-U.S. Holder will be exempt from the U.S. federal withholding tax described above. To claim the exemption, the Non-U.S. Holder must furnish to the applicable withholding agent a valid IRS Form W-8ECI, certifying that the dividends are effectively connected with the Non-U.S. Holder’s conduct of a trade or business within the U.S.

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Any such effectively connected dividends will be subject to U.S. federal income tax on a net income basis at the regular rates applicable to a United States person. A Non-U.S. Holder that is a corporation also may be subject to a branch profits tax at a rate of 30% (or such lower rate specified by an applicable income tax treaty) on its effectively connected earnings and profits (as adjusted for certain items), which will include such effectively connected dividends. Non-U.S. Holders should consult their tax advisors regarding any applicable tax treaties that may provide for different rules.

Sale or Other Taxable Disposition

Subject to the discussion below on backup withholding and the Foreign Account Tax Compliance Act, a Non-U.S. Holder generally will not be subject to U.S. federal income tax on any gain realized upon the sale or other taxable disposition of our common stock unless:

Gain described in the first bullet point above generally will be subject to U.S. federal income tax on a net income basis at the regular rates applicable to a United States person. A Non-U.S. Holder that is a corporation also may be subject to a branch profits tax at a rate of 30% (or such lower rate specified by an applicable income tax treaty) on its effectively connected earnings and profits (as adjusted for certain items), which will include such effectively connected gain.

A Non-U.S. Holder described in the second bullet point above will be subject to U.S. federal income tax at a rate of 30% (or such lower rate specified by an applicable income tax treaty) on any gain derived from the disposition, which may generally be offset by U.S. source capital losses of the Non-U.S. Holder (even though the individual is not considered a resident of the U.S.), provided the Non-U.S. Holder has timely filed U.S. federal income tax returns with respect to such losses.

With respect to the third bullet point above, we believe we currently are not, and do not anticipate becoming, a USRPHC. Because the determination of whether we are a USRPHC depends, however, on the fair market value of our USRPIs relative to the fair market value of our non-U.S. real property interests and our other business assets, there can be no assurance we currently are not a USRPHC or will not become one in the future. Even if we are or were to become a USRPHC, gain arising from the sale or other taxable disposition by a Non-U.S. Holder of our common stock will not be subject to U.S. federal income tax if our common stock is “regularly traded on an established securities market,” as defined by applicable Treasury Regulations, and such Non-U.S. Holder owned, actually and constructively, five percent or less of our common stock throughout the shorter of the five-year period ending on the date of the sale or other taxable disposition or the Non-U.S. Holder’s holding period. If we were to become a USRPHC and our common stock were not considered to be “regularly traded” on an established securities market during the calendar year in which the relevant disposition by a Non-U.S. Holder occurs, such Non-U.S. Holder (regardless of the percentage of common stock owned) would be subject to U.S. federal income tax on a sale or other taxable disposition of our common stock and a 15% withholding tax would apply to the gross proceeds from such disposition.

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Non-U.S. Holders should consult their tax advisors regarding potentially applicable income tax treaties that may provide for different rules.

Information Reporting and Backup Withholding

Payments of dividends on our common stock generally will not be subject to backup withholding, provided the applicable withholding agent does not have actual knowledge or reason to know the Non-U.S. Holder is a United States person and the Non-U.S. Holder either certifies its non-U.S. status, such as by furnishing a valid IRS Form W-8BEN, W-8BEN-E, or W-8ECI, or otherwise establishes an exemption. However, information returns are required to be filed with the IRS in connection with any dividends on our common stock paid to the Non-U.S. Holder, regardless of whether any tax was actually withheld. In addition, proceeds of the sale or other taxable disposition of our common stock within the U.S. or conducted through certain U.S.-related brokers generally will not be subject to backup withholding or information reporting if the applicable withholding agent receives the certification described above and does not have actual knowledge or reason to know that such Non-U.S. Holder is a United States person, or the Non-U.S. Holder otherwise establishes an exemption. If a Non-U.S. Holder does not provide the certification described above or the applicable withholding agent has actual knowledge or reason to know that such Non-U.S. Holder is a United States person, payments of dividends or of proceeds of the sale or other taxable disposition of our common stock generally will be subject to backup withholding at a rate currently equal to 24% of the gross proceeds of such dividend, sale, or taxable disposition. Proceeds of a disposition of our common stock conducted through a non-U.S. office of a non-U.S. broker generally will not be subject to backup withholding or information reporting.

Copies of information returns that are filed with the IRS may also be made available under the provisions of an applicable treaty or agreement to the tax authorities of the country in which the Non-U.S. Holder resides or is established.

Backup withholding is not an additional tax. Any amounts withheld under the backup withholding rules may be allowed as a refund or a credit against a Non-U.S. Holder’s U.S. federal income tax liability, provided the required information is timely furnished to the IRS.

Non-U.S. Holders should consult their tax advisors regarding information reporting and backup withholding.

Additional Withholding Tax on Payments Made to Foreign Accounts

Withholding taxes may be imposed under Sections 1471 to 1474 of the Code (such Sections commonly referred to as the Foreign Account Tax Compliance Act (“FATCA”)) on certain types of payments made to non-U.S. financial institutions and certain other non-U.S. entities. Specifically, a 30% withholding tax may be imposed on dividends on, or gross proceeds from the sale or other disposition of, our common stock paid to a “foreign financial institution” or a “non-financial foreign entity” (each as defined in the Code) (including, in some cases, when such foreign financial institution or non-financial foreign entity is acting as an intermediary), unless (1) the foreign financial institution undertakes certain diligence and reporting obligations, (2) the non-financial foreign entity either certifies it does not have any “substantial United States owners” (as defined in the Code) or furnishes identifying information regarding each direct and indirect substantial United States owner, or (3) the foreign financial institution or non-financial foreign entity otherwise qualifies for an exemption from these rules. If the payee is a foreign financial institution and is subject to the diligence and reporting requirements in (1) above, it must enter into an agreement with the U.S. Department of the Treasury requiring, among other things, that it undertake to identify accounts held by certain “specified United States persons” or “United States owned foreign entities” (each as defined in the Code), annually report certain information about such accounts, and withhold 30% on certain payments to noncompliant foreign financial institutions and certain other account holders. Foreign financial institutions located in jurisdictions that have an intergovernmental agreement with the U.S. governing FATCA may be subject to different rules.

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Under the Code, applicable Treasury Regulations and administrative guidance, withholding under FATCA generally applies to payments of dividends on our common stock. The U.S. Department of the Treasury released proposed regulations (which may be relied upon by taxpayers until final regulations are issued), which eliminate FATCA withholding on the gross proceeds from a sale or other disposition of our common stock. Accordingly, FATCA withholding on the gross proceeds from a sale or other disposition of our common stock is not currently expected to apply.

Prospective investors should consult their tax advisors regarding the potential application of withholding under FATCA to their investment in our common stock.

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PLAN OF DISTRIBUTION

The shares of our common stock covered by this prospectus are being registered to permit the selling stockholders to offer and sell such shares from time to time after the date of this prospectus. We will not receive any of the proceeds from the sale or other distribution of the common stock offered by the selling stockholders.

The selling stockholders may act independently of us in making decisions with respect to the timing, manner, and size of any of their sales. The selling stockholders and certain of their successors, including certain transferees, assignees, and donees (including charitable organizations), may make sales of the shares of common stock covered by this prospectus from time to time through one or more methods specified herein or through a combination of any of such methods or any other method permitted pursuant to applicable law. Such offers and sales may be made directly to purchasers, through underwriters, to dealers or through agents, on NASDAQ or any other stock exchange on which the shares are listed or otherwise at prices and under terms prevailing at the time of the sale, at prices related to the then-current market price, at fixed prices, at varying prices determined at the time of sale, at privately negotiated prices or any other method permitted pursuant to applicable law. Such sales may be effected by a variety of methods, including the following:

The selling stockholders may enter into derivative transactions with third parties or sell securities not covered by this prospectus to third parties in privately negotiated transactions.

A selling stockholder that is an entity may elect to make an in-kind distribution of shares of our common stock to its members, partners or equityholders pursuant to the registration statement of which this prospectus

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forms a part by delivering a prospectus and an applicable prospectus supplement or a free writing prospectus. To the extent that such members, partners or equityholders are not affiliates of ours, such members, partners or equityholders would thereby receive freely tradeable shares of our common stock pursuant to the distribution through a registration statement. To the extent a distributee is an affiliate of ours (or to the extent otherwise required by law), we may file a prospectus supplement or a free writing prospectus in order to permit the distributees to use the prospectus to resell our shares of common stock acquired in the distribution.

The selling stockholders may enter into hedging transactions with broker-dealers or any other person, in connection with such broker dealer or other person who may in turn engage in short sales of the shares of common stock in the course of hedging the positions they assume. The selling stockholders also may sell shares short and deliver shares covered by this prospectus and any applicable prospectus supplement to close out the short positions or loan, pledge, or grant a security interest in, some or all of the shares owned by them to broker-dealers that in turn may sell such shares.

The selling stockholders may also directly make offers to sell some or all of the shares of common stock offered by them pursuant to this prospectus to, or solicit offers to purchase such shares from, purchasers from time to time.

If the selling stockholders use one or more underwriters in the sale, the underwriters will acquire the securities for their own account, and they may resell these securities from time to time in one or more transactions, including negotiated transactions, at a fixed public offering price or at varying prices determined at the time of sale. The securities may be offered and sold to the public either through underwriting syndicates represented by one or more managing underwriters or directly by one or more of such firms. In connection with those sales, underwriters may be deemed to have received compensation from the selling stockholders in the form of underwriting discounts or commissions and may also receive commissions from purchasers of the shares for which they may act as agents. Underwriters may resell the shares to or through dealers, and those dealers may receive compensation in the form of one or more discounts, concessions, or commissions from the underwriters and commissions from purchasers for which they may act as agents.

From time to time, the selling stockholders may sell the shares of common stock offered by them pursuant to this prospectus to one or more dealers acting as principals. The dealers, which may be deemed to be “underwriters” as that term is defined in the Securities Act, may then resell the shares to purchasers.

The selling stockholders may designate broker-dealers as agents from time to time to solicit offers from purchasers to purchase the shares of common stock offered by them pursuant to this prospectus, or to sell such shares in ordinary brokerage transactions, on their behalf. Such broker-dealers may be deemed to be “underwriters” as that term is defined in the Securities Act in such offering.

The selling stockholders or their respective underwriters, broker-dealers, or agents may make sales of the shares of common stock that are deemed to be an at-the-market offering as defined in Rule 415 of the Securities Act, which includes sales of such shares made directly on or through any stock exchange on which the shares are listed, the existing trading market for the shares, or in the over-the-counter market or otherwise.

From time to time, one or more of the selling stockholders may pledge, hypothecate or grant a security interest in some or all of the shares of common stock owned by them. In the event of default, the pledgees, secured parties or persons to whom the shares have been hypothecated will, to the extent registration rights are transferable and are transferred upon foreclosure, be deemed to be selling stockholders under this prospectus.

In addition to the transactions described above, the selling stockholders may sell the shares of common stock offered by them pursuant to this prospectus in compliance with available exemptions from the registration requirements under the Securities Act, rather than pursuant to this prospectus.

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The selling stockholders may decide to sell all or a portion of the securities offered by them pursuant to this prospectus or may decide not to sell any securities under this prospectus. In addition, the selling stockholders may transfer, sell, or dispose of the securities by other means not described in this prospectus.

The selling stockholders and any other persons participating in the sale or distribution of shares of common stock will be subject to applicable provisions of the Exchange Act and the rules and regulations thereunder, including Regulation M. Regulation M may limit the timing of purchases and sales of any of the shares by the selling stockholders and any other such persons. In addition, Regulation M may restrict the ability of any person engaged in the distribution of the shares to engage in market-making activities with respect to the shares being distributed for a period of up to five business days before the distribution. This may affect the marketability of the shares and the ability of any person or entity to engage in market-making activities with respect to the shares.

To the extent required, the securities to be sold, the names of the selling stockholders, the respective purchase prices and public offering prices, the names of any agents, dealers or underwriters and any applicable commissions or discounts with respect to a particular offering will be set forth in an accompanying prospectus supplement or a free writing prospectus or, if appropriate, a post-effective amendment to the registration statement that includes this prospectus.

We may indemnify, in certain circumstances, the selling stockholders against certain liabilities to which they may become subject in connection with the sale of the shares of common stock offered by them pursuant to this prospectus, including liabilities arising under the Securities Act. Each of the selling stockholders may indemnify us in certain circumstances against certain liabilities to which we may become subject in connection with the sale of such shares, including liabilities arising under the Securities Act. We and the selling stockholders may agree to indemnify underwriters, dealers and agents who participate in the distribution of the shares offered by them pursuant to this prospectus against certain liabilities to which they may become subject in connection with the sale of such shares, including liabilities arising under the Securities Act. We have also agreed that if the indemnification described above is held by a court or government agency of competent jurisdiction to be unavailable to any indemnified party or is insufficient to hold them harmless in respect of any losses, then each such indemnifying party, in lieu of indemnifying such indemnified party, shall contribute to the amount paid or payable by such indemnified party as a result of such loss in such proportion as is appropriate to reflect the relative fault of the indemnifying party on the one hand and of such indemnified party on the other in connection with the statements or omissions that resulted in such losses, as well as any other relevant equitable considerations.

Certain of the underwriters and their affiliates may engage in transactions with and may perform services for us or our affiliates in the ordinary course of business.

We may pay the expenses of the registration of the shares of common stock offered and sold by the selling stockholders under the registration statement of which this prospectus forms a part, including, but not limited to, all registration and filing fees, fees and expenses of our counsel and accountants, and to reimburse the selling stockholders for any legal fees and expenses reasonably incurred in connection with defending against certain liabilities. The selling stockholders will pay any underwriting discounts and commissions applicable to the shares sold by the selling stockholders.

A prospectus and accompanying prospectus supplement or free writing prospectus in electronic form may be made available on the websites maintained by the underwriters of a given offering. The underwriters may agree to allocate a number of securities for sale to their online brokerage account holders. Such allocations of securities for internet distributions will be made on the same basis as other allocations. In addition, securities may be sold by the underwriters to securities dealers who resell securities to online brokerage account holders.

In connection with offerings of securities under the registration statement of which this prospectus forms a part and in compliance with applicable law, underwriters, brokers, or dealers may engage in transactions that

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stabilize or maintain the market price of the securities at levels above those that might otherwise prevail in the open market. Specifically, underwriters, brokers, or dealers may over-allot in connection with offerings, creating a short position in the securities for their own accounts. For the purpose of covering a syndicate short position or stabilizing the price of the securities, the underwriters, brokers, or dealers may place bids for the securities or effect purchases of the securities in the open market. Finally, the underwriters may impose a penalty whereby selling concessions allowed to syndicate members or other brokers or dealers for distribution of the securities in offerings may be reclaimed by the syndicate if the syndicate repurchases previously distributed securities in transactions to cover short positions, in stabilization transactions or otherwise. These activities may stabilize, maintain, or otherwise affect the market price of the securities, which may be higher than the price that might otherwise prevail in the open market, and, if commenced, may be discontinued at any time. These transactions may be effected on or through any stock exchange on which the shares are listed, the existing trading market for the shares, or in the over-the-counter market or otherwise.

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LEGAL MATTERS

The validity of the issuance of our common stock offered by this prospectus will be passed upon for us by Kirkland & Ellis LLP, Chicago, Illinois. Certain partners of Kirkland & Ellis LLP are members of a limited partnership that is an investor in one or more investment funds associated with Vista or are investors in the Company. Kirkland & Ellis LLP represents entities affiliated with Vista in connection with legal matters.

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EXPERTS

The financial statements incorporated in this prospectus by reference to the Annual Report on Form 10-K for the year ended December 31, 2022 have been so incorporated in reliance on the report of PricewaterhouseCoopers LLP, an independent registered public accounting firm, given on the authority of said firm as experts in auditing and accounting.

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WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly, and current reports, proxy statements, and other information with the SEC (File No. 001-40557). We have filed with the SEC a registration statement on Form S-3 under the Securities Act to register our common stock being offered by this prospectus. This prospectus, and any document incorporated by reference into this prospectus, which form part of the registration statement, does not contain all of the information included in the registration statement and the attached exhibits. You will find additional information about us and our common stock in the registration statement. References in this prospectus to any of our contracts, agreements, or other documents are not necessarily complete, and you should refer to the exhibits attached to the registration statement for copies of the actual contracts, agreements, or documents.

The SEC maintains an Internet site that contains reports, proxy and information statements and other information regarding registrants that file electronically with the SEC. The address of the site is www.sec.gov. This reference to the SEC’s website is an inactive textual reference only and is not a hyperlink.

We are subject to the reporting, proxy, and information requirements of the Exchange Act, and are required to file periodic reports, proxy statements and other information with the SEC. These periodic reports, proxy statements, and other information are available at the website of the SEC referred to above, as well as on our website, www.integralads.com, as soon as reasonably practicable after we electronically file such material with, or furnish it to, the SEC. This reference to our website is an inactive textual reference only and is not a hyperlink. The contents of, or other information accessible through, our website are not part of this prospectus, and you should not consider the contents of, or other information accessible through, our website in making an investment decision with respect to our common stock.

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INFORMATION INCORPORATED BY REFERENCE

The SEC allows us to “incorporate by reference” into this prospectus the information we provide in other documents filed by us with the SEC. The information incorporated by reference is an important part of this prospectus and the applicable prospectus supplement. We incorporate by reference the following documents that we have filed with the SEC (other than portions of these documents that are deemed to have been furnished and not filed in accordance with SEC rules, including pursuant to Item 2.02 or Item 7.01 of any Current Report on Form 8-K (including any financial statements or exhibits relating thereto furnished pursuant to Item 9.01), unless otherwise indicated therein):

In addition, all documents subsequently filed by us with the SEC under Sections 13(a), 13(c), 14, or 15(d) of the Exchange Act (other than portions of these documents that are deemed to have been furnished and not filed in accordance with SEC rules, including pursuant to Item 2.02 or Item 7.01 of any Current Report on Form 8-K (including any financial statements or exhibits relating thereto furnished pursuant to Item 9.01), unless otherwise indicated therein), until all offerings under the registration statement of which this prospectus is a part are completed or terminated, will be considered to be incorporated by reference into this prospectus and to be a part of this prospectus from the dates of the filing of such documents. The most recent information that we file with the SEC automatically updates and supersedes more dated information. Any statement so updated or superseded will not be deemed, except as so updated or superseded, to constitute a part of this prospectus.

We will provide, without charge, to each person, including any beneficial owner, to whom a copy of this prospectus is delivered, upon written or oral request of such person, a copy of any or all of the reports and documents referred to above that have been incorporated by reference into this prospectus. You should direct requests for those documents to the following address:

Integral Ad Science Holding Corp.

99 Wall Street, #1950,

New York, New York 10005

Attn: Chief Human Resources Officer

(646) 278-4871

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PART II

INFORMATION NOT REQUIRED IN PROSPECTUS

Item 14. Other Expenses of Issuance and Distribution

The following table sets forth the various expenses expected to be incurred in connection with the sale and distribution of the securities being registered hereby. Unless otherwise stated in any prospectus supplement relating to an offering by the selling stockholders, all such expenses, other than underwriting discounts and commissions, will be paid by us.

Item 15. Indemnification of Directors and Officers.

Section 102(b)(7) of the Delaware General Corporation Law, or DGCL, allows a corporation to provide in its certificate of incorporation that a director of the corporation will not be personally liable to the corporation or its stockholders for monetary damages for breach of fiduciary duty as a director, except where the director breached the duty of loyalty, failed to act in good faith, engaged in intentional misconduct or knowingly violated a law, authorized the payment of a dividend or approved a stock repurchase in violation of Delaware corporate law or obtained an improper personal benefit. Our Certificate of Incorporation provides for this limitation of liability.

Section 145 of the DGCL, or Section 145, provides that a Delaware corporation may indemnify any person who was, is or is threatened to be made, party to any threatened, pending or completed action, suit or proceeding, whether civil, criminal, administrative, or investigative (other than an action by or in the right of such corporation), by reason of the fact that such person is or was a director, officer, employee, or agent of such corporation or is or was serving at the request of such corporation as a director, officer, employee, or agent of another corporation or enterprise. The indemnity may include expenses (including attorneys’ fees), judgments, fines and amounts paid in settlement actually and reasonably incurred by such person in connection with such action, suit, or proceeding, provided such person acted in good faith and in a manner such person reasonably believed to be in or not opposed to the corporation’s best interests and, with respect to any criminal action or proceeding, had no reasonable cause to believe that such person’s conduct was illegal. A Delaware corporation may indemnify any persons who are, were or are a party to any threatened, pending, or completed action or suit by or in the right of the corporation by reason of the fact that such person is or was a director, officer, employee, or agent of the corporation or enterprise. The indemnity may include expenses (including attorneys’ fees) actually and reasonably incurred by such person in connection with the defense or settlement of such action or suit, provided such person acted in good faith and in a manner such person reasonably believed to be in or not opposed to the corporation’s best interests, provided that no indemnification is permitted without judicial approval if the officer, director, employee, or agent is adjudged to be liable to the corporation. Where a current or

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former officer or director is successful on the merits or otherwise in the defense of any action referred to above, the corporation must indemnify such person against the expenses which such officer or director has actually and reasonably incurred.

Section 145 further authorizes a corporation to purchase and maintain insurance on behalf of any person who is or was a director, officer, employee, or agent of the corporation or is or was serving at the request of the corporation as a director, officer, employee, or agent of another corporation or enterprise, against any liability asserted against such person and incurred by such person in any such capacity, or arising out of such person’s status as such, whether or not the corporation would otherwise have the power to indemnify such person under Section 145.

Our Bylaws provide that we will indemnify our directors and officers to the fullest extent authorized by the DGCL and that we must also pay expenses incurred in defending any such proceeding in advance of its final disposition upon delivery of an undertaking, by or on behalf of an indemnified person, to repay all amounts so advanced if it should be determined ultimately that such person is not entitled to be indemnified under this section or otherwise.

We have entered into indemnification agreements with each of our executive officers and directors. The indemnification agreements provide the executive officers and directors with contractual rights to indemnification, expense advancement and reimbursement, to the fullest extent permitted under the DGCL.

The indemnification rights set forth above shall not be exclusive of any other right which an indemnified person may have or hereafter acquire under any statute, provision of our Certificate of Incorporation or Bylaws, agreement, vote of stockholders or disinterested directors, or otherwise.

We maintain standard policies of insurance that provide coverage (1) to our directors and officers against loss arising from claims made by reason of breach of duty or other wrongful act and (2) to us with respect to indemnification payments that we may make to such directors and officers.

Item 16. Exhibits.

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Item 17. Undertakings

The undersigned registrant hereby undertakes:

provided, however, that paragraphs (i), (ii), and (iii) do not apply if the information required to be included in a post-effective amendment by those paragraphs is contained in reports filed with or furnished to the SEC by the registrant pursuant to Section 13 or Section 15(d) of the Exchange Act that are incorporated by reference in the registration statement, or is contained in a form of prospectus filed pursuant to Rule 424(b) that is part of the registration statement.

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Insofar as indemnification for liabilities arising under the Securities Act may be permitted to directors, officers and controlling persons of the registrant pursuant to the foregoing provisions, or otherwise, the registrant has been advised that in the opinion of the SEC such indemnification is against public policy as expressed in the Securities Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the registrant of expenses incurred or paid by a director, officer, or controlling person of the registrant in the successful defense of any action, suit, or proceeding) is asserted by such director, officer, or controlling person in connection with the securities being registered, the registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the Securities Act and will be governed by the final adjudication of such issue.

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SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, the registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-3 and has duly caused this registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of New York, State of New York, on May 9, 2023.

POWER OF ATTORNEY

The undersigned directors and officers of Integral Ad Science Holding Corp. hereby appoint each of Lisa Utzschneider and Tania Secor, as attorney-in-fact for the undersigned, with full power of substitution and resubstitution, for and in the name, place and stead of the undersigned, to sign and file with the Securities and Exchange Commission under the Securities Act of 1933 any and all amendments (including post-effective amendments) and exhibits to this registration statement on Form S-3 (or any other registration statement for the same offering that is to be effective upon filing pursuant to Rule 462(b) under the Securities Act of 1933) and any and all applications and other documents to be filed with the Securities and Exchange Commission pertaining to the registration of the securities covered hereby, with full power and authority to do and perform any and all acts and things whatsoever requisite and necessary or desirable, hereby ratifying and confirming all that said attorney-in-fact, or his or her substitute or substitutes, may lawfully do or cause to be done by virtue hereof.

Pursuant to the requirements of the Securities Act of 1933, this Registration Statement has been signed by the following persons in the capacities and on the dates indicated.

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Exhibit 5.1

May 9, 2023

Integral Ad Science Holding Corp.

99 Wall Street, #1950

New York, NY 10005

Re: Registration Statement on Form S-3ASR

Ladies and Gentlemen:

We are issuing this opinion in our capacity as special legal counsel to Integral Ad Science Holding Corp., a Delaware corporation (the “Company”), in connection with the preparation of the Registration Statement on Form S-3ASR filed with the Securities and Exchange Commission (the “Commission”) on or about the date hereof under the Securities Act of 1933, as amended (the “Act”) (such Registration Statement on Form S-3ASR, as amended or supplemented, and the documents incorporated by reference therein, is hereinafter referred to as the “Registration Statement”). The Registration Statement relates to the sale from time to time, pursuant to Rule 415 of the General Rules and Regulations promulgated under the Act of an unspecified amount of the Company’s common stock, par value $0.001 per share (the “Common Stock”) that may be offered and sold in one or more offerings from time to time on a delayed or continuous basis by certain stockholders of the Company (the “Selling Stockholders”). The shares of Common Stock that may be offered by the Selling Stockholders are referred to herein as the “Shares”, and includes the currently outstanding shares of Common Stock that may be offered and sold by the Selling Stockholders.

In connection therewith, we have examined originals, or copies certified or otherwise identified to our satisfaction, of such documents, corporate records and other instruments as we have deemed necessary for the purpose of this opinion, including (i) the corporate and organizational documents of the Company, including the Certificate of Incorporation of the Company filed as Exhibit 3.1 to the Registration Statement (the “Certificate of Incorporation”), (ii) minutes and records of the corporate proceedings of the Company and (iii) the Registration Statement and the exhibits thereto.

Austin Bay Area Beijing Boston Brussels Chicago Dallas Hong Kong Houston London Los Angeles Munich New York Paris Salt Lake City Shanghai Washington, D.C.

Integral Ad Science Holding Corp.

May 9, 2023

Page 2

For purposes of this opinion, we have assumed the authenticity of all documents submitted to us as originals, the conformity to the originals of all documents submitted to us as copies and the authenticity of the originals submitted to us as copies. We have also assumed the legal capacity of all natural persons, the genuineness of the signatures of persons signing all documents in connection with which this opinion is rendered, the authority of such persons signing on behalf of the parties thereto other than the Company and the due authorization, execution and delivery of all documents by the parties thereto other than the Company. We have not independently established or verified any facts relevant to the opinions expressed herein, but have relied upon statements and representations of officers and other representatives of the Company and others.

Based upon and subject to the assumptions, qualifications and limitations identified in this opinion, we are of the opinion that the Shares have been duly authorized, validly issued and fully paid and are non-assessable.

Our opinion expressed above is subject to the qualifications that we express no opinion as to the applicability of, compliance with, or effect of any laws except the General Corporation Law of the State of Delaware (including the statutory provisions, all applicable provisions of the Delaware constitution and reported judicial decisions interpreting the foregoing).

For purposes of rendering our opinion expressed above, we have assumed that (i) the Registration Statement remains effective during the offer and sale of the Shares, (ii) at the time of the sale and delivery of each Share (x) there will not have occurred any change in law affecting the validity, legally binding character or enforceability of such Share and (y) the sale and delivery of such Share, the terms of such Share and compliance by the Company with the terms of such Share will not violate any applicable law, any agreement or instrument then binding upon the Company or any restriction imposed by any court or governmental body having jurisdiction over the Company.

We hereby consent to the filing of this opinion with the Commission as Exhibit 5.1 to the Registration Statement. We also consent to the reference to our firm under the heading “Legal Matters” in the Registration Statement. In giving this consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Commission.

We do not find it necessary for the purposes of this opinion, and accordingly we do not purport to cover herein, the application of the securities or “Blue Sky” laws of the various states to the sale of the Shares.

This opinion is limited to the specific issues addressed herein, and no opinion may be inferred or implied beyond that expressly stated herein. This opinion speaks only as of the date that the Registration Statement becomes effective under the Act, and we assume no obligation to revise or supplement this opinion after the date of effectiveness should the General Corporation Law of the State of Delaware be changed by legislative action, judicial decision or otherwise after the date hereof.

This opinion is furnished to you in connection with the filing of the Registration Statement and is not to be used, circulated, quoted or otherwise relied upon for any other purposes.

Integral Ad Science Holding Corp.

May 9, 2023

Page 3

Exhibit 23.1

CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

We hereby consent to the incorporation by reference in this Registration Statement on Form S-3 of Integral Ad Science Holding Corp. of our report dated March 2, 2023 relating to the financial statements, which appears in Integral Ad Science Holding Corp.’s Annual Report on Form 10-K for the year ended December 31, 2022. We also consent to the reference to us under the heading “Experts” in such Registration Statement.

Exhibit 107

Calculation of Filing Fee Table

Form S-3ASR

(Form Type)

Integral Ad Science Holding Corp.

(Exact Name of Registrant as Specified in its Charter)

Table 1 – Newly Registered Securities and Carry Forward Securities