The 19-year-old Sunnyvale, Calif., Internet pioneer that has been struggling to remain relevant and the 7-year-old New York startup started serious talks last week, culminating in the Yahoo! board's vote to approve the all-cash deal Sunday morning, The New York Times said.
Tumblr Inc.'s board also approved the deal, The Wall Street Journal said.
Neither company had an immediate comment.
Tumblr, which claims 108.5 million blogs reaching 44 million people in the United States and 134 million around the world, would give Yahoo! a thriving social-networking and communications hub, analysts said.
Tumblr is more popular with younger adults than Yahoo!, whose customer base generally skews older, the Journal said. Tumblr is also growing more quickly on smartphones than Yahoo!
But Tumblr has not shown it can be profitable, the Times and Journal said.
Tumblr founder David Karp, 26, who dropped out of high school as a 15-year-old programmer, is expected to get nearly $250 million from the deal, the Times said.
It was not immediately clear how many of Tumblr's 175 employees would move over to Yahoo!