WebM is a video file format. It is primarily intended to offer a royalty-free alternative to use in the HTML5 video tag. It has a sister project WebP. The development of the format is sponsored by Google, and the corresponding software is distributed under a BSD license.

WebM initially supported VP8 video and Vorbis audio streams. In 2013 it was updated to accommodate VP9 video and Opus audio.

Native WebM support by Mozilla Firefox, Opera and Google Chrome was announced at the 2010 Google I/O conference. Internet Explorer 9 requires third-party WebM software. Safari for Windows and Mac OS X relies on QuickTime to play web media, which as of 1 April 2011, does not support WebM unless a third-party plug-in is installed. In January 2011, Google announced that the WebM Project Team will release plugins for Internet Explorer and Safari to allow playback of WebM files through the standard HTML5 <video> tag. As of 9th June 2012, a public preview version of this plugin is available for Internet Explorer 9. VLC media player, MPlayer and K-Multimedia Player have native support for playing WebM files.

YouTube offers WebM videos as part of its HTML5 player. All uploaded files are encoded into WebM in 360p, 480p, 720p and 1080p resolutions. YouTube has committed to encode its entire portfolio of videos to WebM. The YouTube app for the Nintendo Wii uses WebM for streaming videos or H.263 as a fall-back option.