Sunnyvale, California- Yahoo has completed its $1.1 billion acquisition of online blogging forum Tumblr.

The move represents Yahoo CEO Marissa Mayer's boldest move since she left Google a little less than a year ago to lead Yahoo's comeback.

It also marks Yahoo's most expensive acquisition since the Sunnyvale, Calif., company bought online search engine Overture for $1.3 billion 10 years ago.

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

Under the terms originally announced in May, Tumblr will remain independently operated and Karp will stay CEO.