Yahoo is buying online blogging forum Tumblr for $US1.1 billion ($A1.13 billion) as chief executive Marissa Mayer tries to rejuvenate an internet icon that has fallen behind the times.

The deal, announced on Monday, represents Mayer’s boldest move yet since she left Google 10 months ago to lead Yahoo’s latest comeback attempt.

It marks Yahoo’s most expensive acquisition since the Sunnyvale, California, company bought Overture for $US1.3 billion ($A1.34 billion) a decade ago.

Tumblr now figures to play a pivotal role in Mayer’s attempt to reshape Yahoo Inc.

Mayer is betting that Tumblr, a six-year-old service started by high school dropout David Karp, will provide Yahoo with a hook to reel in more traffic and advertisers on smartphones and tablets.

The companies say Tumblr will remain independently operated and Karp will stay as chief executive.