Athletic Club; (Basque: Athletic Kluba), also commonly known as Athletic de Bilbao; (Basque: Bilboko Athletic Kluba), is a professional football club, based in Bilbao, Basque Country, Spain.

They are known as Los Leones (The Lions) because their stadium was built near a church called San Mamés (Saint Mammes). Mammes was a semi-legendary early Christian thrown to the lions by the Romans. Mammes pacified the lions and was later made a saint.

The club is one of three founding members of the Primera División that have never been relegated from the top division, the others being Real Madrid and Barcelona. Athletic have won the league on eight occasions and in the historical classification of La Liga, Athletic are in fourth place. In the table of Copa del Rey titles, Athletic is second only to Barcelona, having won the Cup 23 times. The club also has a women's team, which has won four championships in the Primera División Femenina.

The club is known for its cantera policy of bringing young Basque players through the ranks, as well as recruiting top Basque players from other clubs (like Joseba Etxeberria and Javi Martínez). Athletic official policy is signing professional players native to or trained in football in the greater Basque Country, including Biscay, Gipuzkoa, Álava and Navarre (in Spain); and Labourd, Soule and Lower Navarre (in France). Since its foundation, Athletic has played exclusively with players meeting the criteria to be deemed as Basque players, and has been one of the most successful teams of La Liga. This can be seen as a unique case in European football; it has gained Athletic both admirers and critics. The club has been praised for promoting home grown players and club loyalty. Athletic is one of only three professional clubs in Spain in La Liga (the others being Real Madrid, and Barcelona) that is not a sports corporation; it is owned and operated by its club members.

Athletic Club Emakumeen Futbol Taldea is the women's football (soccer) section of Athletic Bilbao, currently competing in the Spanish First Division. It plays in the club's facilities in Lezama.

The team was originally founded in 2000 as Leioa EFT after CD Sondika disbanded its women's team, which had played in the defunct División de Honor in the 1990s. After the new team gained promotion to the new Superliga Femenina in just two years it was absorbed by Athletic Bilbao. In its debut season Athletic won the championship, and it successfully defended the title in the next two seasons, becoming the first team to win the trophy in property. After a disappointing 2006 season, Athletic won its fourth title in 2007. They made consequently four appearances in the UEFA Women's Cup, where it was knocked out in the group stages by 1. FFC Frankfurt in 2004, Djurgårdens IF in 2005, SV Saestum in 2006 and ASDCF Bardolino in 2008.

Athletic was third in the next four championships. In 2012 it was the runner-up, and it reached the national cup's final for the first time, losing to RCD Espanyol after extra time. On 5 January 2013 the team celebrated its tenth anniversary beating Arsenal LFC in a friendly match.