|Tumblr's founder David Karp will keep his role as chief executive|
Courtesy of BBC News
Yahoo's chief executive Marissa Mayer said that as part of its promise "not to screw it up", Tumblr would operate independently.
The deal is the largest made by Ms Mayer since she took the helm at Yahoo last July, and she described the acquisition as a "unique opportunity."
"On many levels, Tumblr and Yahoo couldn't be more different, but at the same time, they couldn't be more complementary," added Ms Mayer.
Mr Karp, 26, who owns 25% of the privately-owned company he co-founded with Marco Arment in 2007, said he was "elated" to have the support of Yahoo.
"Tumblr gets better faster with more resources to draw from," he added. Mr Karp emphasised that neither its aims or team was changing as a result of Yahoo's purchase.
Tumblr's 2012 revenue was just $13m, according to a report by Forbes magazine, leading analysts to suggest Yahoo had overpaid for the deal.
"Even if revenue was $100 million, it means Yahoo paid 10 times revenue," said BGC Financial analyst Colin Gillis. "Ten times is what you pay to date the belle of the ball. It's on the outer bands of M&A."
Tumblr combines elements of blogging with social networking, and its simple design has attracted millions of users since its launch.
According to its homepage, it now hosts 108 million blogs, with a total of 50.7 billion posts.
It also has a significant presence on mobile devices.
But despite its fast-growing user base, it has struggled to make money and has traditionally resisted advertising.
It said in April 2012 that it would roll out limited use of adverts.
Ms Mayer said Yahoo would now work with Tumblr to create ads that "are seamless and enhance the user experience".