The Wikimedia Foundation, Inc. (WMF) is an American 501(c)(3) nonprofit organization headquartered in San Francisco, California, and registered there as a charitable foundation. It is the host of Wikipedia, the eighth most visited website in the world. It also hosts fourteen related open collaboration projects, and supports the development of MediaWiki, the wiki software which underpins them all. The foundation was established in 2003 in St. Petersburg, Florida by Jimmy Wales, as a non-profit way to fund Wikipedia and other wiki projects which had previously been hosted by Bomis, Wales' for-profit company.

The Wikimedia Foundation provides the technical and organizational infrastructure to enable members of the public to develop wiki-based content in languages across the world. The foundation does not write or curate any of the content on the projects themselves. Instead, this is done by volunteer editors, such as the Wikipedians. However, it does collaborate with a network of individual volunteers and affiliated organizations, such as Wikimedia chapters, thematic organizations, user groups and other partners.

The foundation finances itself mainly through millions of small donations from readers and editors, collected through email campaigns and annual fundraising banners placed on Wikipedia and its sister projects. These are complemented by grants from philanthropic organizations and tech companies, and starting in 2022, by services income from Wikimedia Enterprise. As of 2023, it has employed over 700 staff and contractors, with net assets of $255 million and an endowment which has surpassed $100 million.