Twitter said Monday that it had struck a deal to acquire TapCommerce, a two-year-old startup that automates buying for app advertising. The price was around $100 million, according to a person familiar with the deal.

The acquisition comes on the heels of Twitter's purchase of Namo Media, a company focused on native mobile ads, on June 5. Last September, Twitter agreed to acquire the mobile-ad exchange MoPub for $300 million.

With these deals, Twitter is focusing intently on bulking up in programmatic ad technology and expanding its mobile advertising reach and repertoire. Richard Alfonsi, Twitter's VP-global online sales, called this its "off-Twitter strategy."

"They continue the ad strategy we laid out with the MoPub acquisition," he said of TapCommerce. "This provides a super set of inventory in the mobile ecosystem."

TapCommerce currently buys ads through real-time bidding across 50,000 different apps, according to Twitter, adding to the "tens of thousands" of sites and apps reached by MoPub, by Twitter's count.

Founded in New York in 2012, TapCommerce specializes in mobile-app "re-targeting" -- serving ads to smartphone owners that have already installed apps but left them largely unused. The company relies on "deep linking," which send users swiftly from one app to another, a technology that Facebook and Google have been advancing recently.

TapCommerce works with travel and e-commerce companies, like eBay, as well as mobile game developers. It has raised $11.7 million in venture capital funding.

The acquisition is also part of Twitter's aggressive foray into the app-install business, Facebook's cash cow. Earlier on Monday, Twitter introduced new ad formats for app-install ads, along with a new cost-per-click pricing structure. The TapCommerce business will be folded into these efforts, which are built primarily to capture direct-response ad dollars, Mr. Alfonsi said.

With its re-targeting ads, TapCommerce has worked extensively with Facebook inventory. Asked whether or how TapCommerce would adjust its relationship to Facebook, Mr. Alfonsi said that was still to be determined.

In a blog post, TapCommerce CEO Brian Long addressed his existing ad buyers, writing, "your TapCommerce experience will not change."

Last quarter, Twitter reported ad revenue of $250 million, with 80% coming from mobile.