Yahoo did not disclose how much it paid for Xobni which is 'inbox' spelled backwards but said it planned to use the startup's technology to improve Yahoo! Mail, Messenger and other "communications offerings."
San Francisco-based Xobni was launched in 2006, and two years later the venture-backed firm hired Yahoo executive Jeff Bonforte as chief.
A message posted at the Xobni blog on Wednesday indicated that the team was settling in to new digs at Yahoo's main campus in the Silicon Valley city of Sunnyvale.
"This is just the type of gig we've been preparing for throughout our history," Xobni said.
"Soon, you'll be able to use Yahoo! products with Xobni goodness baked right in."
Xobni products help manage email and other communications based on patterns of exchanges between people and their contacts.
The acquisition continues a shopping spree launched after Marissa Mayer became chief of Yahoo a year ago.
Yahoo on Tuesday announced it bought Qwiki, a New York operation behind an application that converts video and pictures on iPhones into sharable movie clips complete with music soundtracks.
The Qwiki buy announcement came a day after Yahoo put out word that it bought Bignoggins Productions, a one-person operation specializing in fantasy sports applications for iPhones.
Yahoo in June completed a billion-dollar deal taking over the popular blogging platform Tumblr, a move aimed at bringing more youthful users into the company's orbit.
Yahoo last month bought startup GhostBird Software, which makes picture-taking applications for iPhones, to enhance its Flickr photo service.
Since former Google executive Mayer became chief at Yahoo, the company has racked up a series of acquisitions including startups Alike, Stamped, Snip.it and a Summly application built by a British teen.
Yahoo recently bought PlayerScale, a startup that powers games played on smartphones, tablets, consoles or personal computers.
"When a CEO is in trouble, and innovation is not happening internally, they go acquisition crazy," independent Silicon Valley analyst Rob Enderle said while discussing the pace at which Yahoo is snapping up startups.
"Marissa Mayer is bouncing all over the place; it looks like she is in a panic."
Making acquisitions in rapid succession also burns through capital, meaning Yahoo must be careful not to eat away too much of its cash reserves or it could wind up "on death watch," according to the analyst.
Yahoo is reported to be eyeing online video website Hulu after its bid to buy a majority stake in France-based Dailymotion was derailed in May.
Mayer's plan for reviving the fortunes of the faded Internet pioneer includes making priorities of mobile devices, video, personalized digital content, and elevating the company's popularity outside the United States.