Microsoft Cortana is an intelligent personal assistant on the Windows Phone 8.1 operating system available on an opt-in basis. Its name comes from Cortana, the artificial intelligence character in the Halo series, with Jen Taylor, the character's voice actress, returning to voice the personal assistant.

Cortana was demonstrated for the first time by Microsoft corporate vice president Joe Belfiore at the Microsoft BUILD Developer Conference (April 2-4, 2014) in San Francisco.

It has been launched as a key ingredient of Microsoft's planned "makeover" of the future operating systems for Windows Phone, Windows, as well as Xbox One. It is currently available to all users of Windows Phone 8.1 in the United States as a beta, and will eventually roll out to all WP8.1 users worldwide in late 2014 and early 2015.