Private-equity firm Vista Equity Partners has agreed to acquire a majority stake in Integral Ad Science, the latest in a wave of deals involving advertising technology companies that help marketers measure the effectiveness of their digital ads.

Founded in 2009 as AdSafe Media, IAS works with companies such as Verizon Communications Inc., HP Inc. and Citibank to help prove that their digital ads are viewable, meaning they aren’t running on parts of websites that can’t be seen.

The company also helps marketers ensure their ads aren’t running on sites with objectionable content.

“We feel like there’s so much more we could be doing for the industry, both for buyers of advertising and sellers of advertising,” said IAS Chief Executive Scott Knoll. Vista provides IAS with the “operational excellence, intellectual capital and resources” to help the company “measure more types of media in more countries,” he said.

IAS touts about 600 employees in various markets, with more than $140 million in revenue, according to a person close to the company.

IAS investors include Sapphire Ventures, Pelion Venture Partners, August Capital and Accomplice.

Vista is already a big investor in ad and marketing tech companies. In 2016, it bought marketing technology firm Marketo for $1.8 billion and a year earlier acquired a majority stake in advertising software company Mediaocean in a deal that valued the company at roughly $720 million.

Competitors to IAS in the ad measurement and verification business have also been attracting financial backing. Last summer, private-equity firm Providence Equity Partners LLC invested about $200 million in DoubleVerify, valuing the company at more than $300 million. Months earlier, Oracle acquired Moat, a digital measurement company known for its role in auditing ads across platforms like Facebook, Google’s YouTube and Snapchat.

Third-party measurement firms like DoubleVerify, Moat and Integral Ad Science work with large brands and publishers to ensure ads are clicked on by real people, versus computerized “bots” that mimic human behavior, and that they appear in appropriate media content.

The transaction between IAS and Vista is expected to close in the third quarter this year.

“We are thrilled to partner with the IAS team to help the company accelerate investments that boost innovation and growth to meet the needs of customers,” said Brian Sheth, co-founder and president of Vista Equity Partners in a statement. “Brand safety, viewability, and ad fraud are serious challenges to marketers and publishers around the globe.”

—Suzanne Vranica contributed to this article.

Write to Alexandra Bruell at alexandra.bruell@wsj.com