Tumblr (stylized in its logo as tumblr.) is a microblogging platform and social networking website founded by David Karp and owned by Yahoo! Inc. The service allows users to post multimedia and other content to a short-form blog. Users can follow other users' blogs, as well as make their blogs private. Much of the website's features are accessed from the "dashboard" interface, where the option to post content and posts of followed blogs appear.

As of May 1, 2014, Tumblr hosts over 184 million blogs. The company's headquarters is in New York City.

Yahoo! announced its intention to acquire Tumblr on May 20, 2013, for approximately $1.1 billion. The deal closed on June 20, 2013.