SAN FRANCISCO - Yahoo is buying online blogging forum Tumblr for $1.1 billion as CEO Marissa Mayer tries to rejuvenate an Internet icon that had fallen behind the times.

The deal announced Monday is Mayer's boldest move 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 online search engine Overture a decade ago for $1.3 billion in cash and stock.