Bayer 04 Leverkusen Fußball GmbH, also known as Bayer 04 Leverkusen ([ˌbaɪ̯ɐ ˈleːvɐˌkuːzn̩]), Bayer Leverkusen, or simply Leverkusen, is a professional football club based in Leverkusen in the state of North Rhine-Westphalia. The club competes in the Bundesliga, the top tier of German football, and plays its home matches at the BayArena.

The club was founded in 1904 by employees of the German pharmaceutical company Bayer AG, whose headquarters are in Leverkusen and from which the club draws its name. It was formerly the best-known department of TSV Bayer 04 Leverkusen, a sports club whose members also participate in athletics, gymnastics, basketball, field handball and other sports including the RTHC Bayer Leverkusen (rowing, tennis and hockey). In 1999, the football department was separated from the sports club and is now a separate entity formally called Bayer 04 Leverkusen Fußball GmbH. Bayer were first promoted to the Bundesliga in 1979, and have remained in the top division ever since. The club's main colours are red and black, both having been used as the main shirt colour and with red and black stripes also having been used as home colours.

Bayer Leverkusen have finished as runner-up in the Bundesliga five times without winning the competition, a record in German football. The club has won one DFB-Pokal and one UEFA Cup. Bayer also finished runner-up in the 2001–02 UEFA Champions League, falling 2–1 to Real Madrid in the final. Their local rivals are 1. FC Köln.