SUNNYVALE -- Yahoo (YHOO) on Thursday announced its first acquisition under CEO Marissa Mayer, a New York-based mobile startup named Stamped that was founded by two former Google (GOOG) employees and received financing from celebrities including Justin Bieber.

The acquisition was first reported by technology blogs Mashable and SAI, but Mayer verified it with an Instagram photo from Stamped headquarters showing the Yahoo CEO and the three co-founders of Stamped. "Got to visit our new acquisition, Stamped, this morning -- happy to be reunited with Robby (@rmstein) and his team," Mayer wrote with the photograph.

"Robby" is Robby Stein, the CEO of Stamped who once worked under Mayer as a product manager at Google after also working on Gmail. He co-founded the company with Bart Stein, who previously worked at Google Creative Lab, and Kevin Palms. The mobile app allows users to "stamp" restaurants and other businesses or media they enjoy for friends to see and share.

The acquisition was also announced later Thursday in a blog post titled "Accelerating Yahoo's Mobile Talent" from Adam Cahan, Yahoo's senior vice president of emerging products and technology. "We're thrilled to announce that we've acquired a very talented team based in New York City to help us create a new center of mobile product development for Yahoo."

Mayer, 37, was employee No. 20 at Google and a high-ranking executive when she surprised Silicon Valley by agreeing to take over Yahoo last summer. The Sunnyvale company has struggled to stay relevant while facing strong competition from Google and Facebook, which have both surpassed the former king of online display advertising in the important revenue-generating arena.

In her first earnings report conference call Monday, Mayer said the company would be looking for small acquisitions, with a focus on the mobile sector, where she said Yahoo was behind rivals.

"At some point at least half our technical workforce should be working on mobile," Mayer said in the call, "because Yahoo should be a predominantly mobile company."

Stamped lists seven employees on its website, not including an intern, along with its three co-founders. An acquisition price was not disclosed. The company received startup funds from Google Ventures and Bain Capital Ventures before relaunching with a second round of funding from celebrities that included tween-pop idol Justin Bieber, television hosts Ryan Seacrest and Ellen DeGeneres, The New York Times, Google Chairman and former CEO Eric Schmidt, and celebrity chef Mario Batali.

Contact Jeremy C. Owens at 408-920-5876; follow him at Twitter.com/mercbizbreak.