Knol was a Google project that aimed to include user-written articles on a range of topics. The lower-case term knol, which Google defined as a "unit of knowledge", referred to an article in the project. Knol was often viewed as a rival to Wikipedia.

The project was led by Udi Manber, a Google vice president of engineering. It was announced on December 13, 2007, and was opened in beta version on July 23, 2008, with a few hundred articles, mostly in the health and medical field.

Knol did not find a significant audience and became viewed as a failure. The project was closed on April 30, 2012, and all content was deleted after October 1, 2012. The Internet Archive has snapshots of Knol archived between July 2008 and May 2012.

Any contributor could create and own new Knol articles, and there could be multiple articles on the same topic with each written by a different author.

Authors could also choose to include ads from Google's AdSense on their pages. This profit-sharing was criticized as incentivizing self-promotion or spam.

All contributors to the Knol project had to sign in with a Google account and were supposed to state their real names. Contributions were licensed by default under the Creative Commons CC-BY-3.0 license (which allowed anyone to reuse the material as long as the original author was named), but authors were also able to choose the CC-BY-NC-3.0 license (which prohibits commercial reuse) or traditional copyright protection instead. Knol employed "nofollow" outgoing links, using an HTML directive to prevent links in its articles from influencing search-engine rankings.

Knol was described both as a rival and as a complement to Wikipedia, offering a different format that addressed many of Wikipedia's shortcomings. BBC News reported that "Many experts saw the initiative as an attack on the widely used Wikipedia communal encyclopaedia." The non-profit Wikimedia Foundation, which owned the name Wikipedia and the servers hosting the Wikipedia projects, welcomed the Google Knol initiative, saying that "The more good free content, the better for the world." While Wikipedia articles were written collectively under a "neutral point of view" policy, Knol aimed to highlight personal expertise by emphasizing authorship.

After Knol's beta launch, Google product manager Cedric Dupont responded to the idea that Google intended Knol to be a "Wikipedia killer" by saying, "Google is very happy with Wikipedia being so successful. Anyone who tries to kill them would hurt us." The New York Times noted similarities in design between Knol and Wikipedia, such as use of the same font. Dupont responded that the use was simply a coincidence as it is a commonly used font.