Twitter co-founder Evan Williams told technology blog Tech Crunch that the size of the transaction was not being disclosed, but was paid "mostly in stock".
Summize crawls online reviews and blogs to create summarized reviews of music, movies, books and other media. It provides search listings for the most reviewed and most popular products; recent blog mentions; YouTube videos; and sites linked to through blogs.
Discussions between the tech companies had been underway for months. Summize helped Twitter last month to keep the platform stable during the launch of the Apple 3G iPhone.
Twitter has hired five out of six Summize employees, all engineers, who will move to work at Twitter in San Francisco. The search engine's chief technology officer Greg Pass has been appointed as Twitter's director of engineering and operations.
Williams said Twitter was "really impressed by the quality of their user interface, the quality of their search and a bunch of features", and was "super-impressed with their engineering, [and] with their culture".
Summize's founder and chief executive officer Jay Verdy is developing other start-ups and is not joining Twitter.
The deal, which had been under discussion for some time, was closed in the last week.