1. Fußball-Club Kaiserslautern e. V., also known as 1. FCK, FCK (German pronunciation: [ˌɛftseːˈkaː] ), FC Kaiserslautern (pronounced [ˌɛftseː kaɪzɐsˈlaʊtɐn] ), K'lautern or colloquially Lautern (pronounced [ˈlaʊtɐn] ), is a German sports club based in Kaiserslautern, Rhineland-Palatinate. In addition to football, the club also operates in several other sports.

On 2 June 1900, Germania 1896 and FG Kaiserslautern merged to create FC 1900. In 1909, the club went on to join FC Palatia (founded in 1901) and FC Bavaria (founded in 1902) to form FV 1900 Kaiserslautern. In 1929, they merged with SV Phönix to become FV Phönix-Kaiserslautern, before finally taking on their current name in 1933.

As a founding member of the Bundesliga, FCK played from 1963 to 1996 uninterrupted in the top division. They have won four German championships, two DFB-Pokals, and one DFL-Supercup, and rank among the most successful football clubs in Germany, currently occupying eleventh place in the all-time Bundesliga table. The club's international performances include reaching the Champions League quarter-finals in 1999, as well as playing in the UEFA Cup semi-finals twice. Their first league title in the Bundesliga era was won in 1991. Kaiserslautern then won the German championship in the 1997–98 season, becoming the only team ever to win as a newly promoted team. After a six-year spell in the second tier, in 2018 they were relegated to the 3. Liga for the first time. In 2022, Kaiserslautern was promoted again to the 2. Bundesliga after winning the promotion playoff match.

Since 1920, Kaiserslautern's stadium has been the Fritz-Walter-Stadion, named in 1985 after Fritz Walter, the captain of the West Germany national team who won the World Cup in 1954. Walter spent his entire career at Kaiserslautern.