Kazaa Media Desktop (/kəˈzɑː/ ka-ZAH; (once stylized as "KaZaA", but later usually written "Kazaa") was a peer-to-peer file sharing application using the FastTrack protocol licensed by Joltid Ltd. and operated as Kazaa by Sharman Networks. Kazaa was subsequently under license as a legal music subscription service by Atrinsic, Inc., which lasted until August 2012.

Kazaa Media Desktop was commonly used to exchange MP3 music files and other file types, such as videos, applications, and documents over the Internet. The Kazaa Media Desktop client could be downloaded free of charge; however, it was bundled with adware and for a period there were "No spyware" warnings found on Kazaa's website. During the years of Kazaa's operation, Sharman Networks and its business partners and associates were the target of copyright-related lawsuits, related to the content distributed via Kazaa Media Desktop on the FastTrack protocol.

By August 2012, the Kazaa website was no longer active.

Kazaa and FastTrack were originally created and developed by Estonian programmers from BlueMoon Interactive including Jaan Tallinn and sold to Swedish entrepreneur Niklas Zennström and Danish programmer Janus Friis (who were later to create Skype and later still Joost and Rdio). Kazaa was introduced by the Dutch company Consumer Empowerment in March 2001, near the end of the first generation of P2P networks typified by the shutdown of Napster in July 2001. Skype itself was based on Kazaa's P2P backend, which allowed users to make a call by directly connecting them with each other.

Initially, some users of the Kazaa network were users of the Morpheus client program, formerly made available by MusicCity. Eventually, the official Kazaa client became more widespread. In February 2002, when Morpheus developers failed to pay license fees, Kazaa developers used an automatic update ability to shut out Morpheus clients by changing the protocol. Morpheus later became a client of the gnutella network.

Consumer Empowerment was sued in the Netherlands in 2001 by the Dutch music publishing body, Buma/Stemra. The court ordered Kazaa's owners to take steps to prevent its users from violating copyrights or else pay a heavy fine. In October 2001 a lawsuit was filed against Consumer Empowerment by members of the music and motion picture industry in the US. In response Consumer Empowerment sold the Kazaa application to Sharman Networks, headquartered in Australia and incorporated in Vanuatu. In late March 2002, a Dutch court of appeal reversed an earlier judgment and stated that Kazaa was not responsible for the actions of its users. Buma/Stemra lost its appeal before the Dutch Supreme Court in December 2003.

In 2003, Kazaa signed a deal with Altnet and Streamwaves to try to convert users to paying, legal customers. Searchers on Kazaa were offered a free 30-second sample of songs for which they were searching and directed to sign up for the full-featured Streamwaves service.

However, Kazaa's new owner, Sharman, was sued in Los Angeles by the major record labels and motion pictures studios and a class of music publishers. The other defendants in that case (Grokster and MusicCity, makers of the Morpheus file-sharing software) initially prevailed against the plaintiffs on summary judgment (Sharman joined the case too late to take advantage of that ruling). The summary judgment ruling was upheld by the Ninth Circuit Court of Appeals, but was unanimously reversed by the US Supreme Court in a decision titled MGM Studios, Inc. v. Grokster, Ltd.