Chelsea Football Club is a professional association football club based in Fulham, West London, England. They compete in the Premier League, the top tier of English football, and rank among the most successful clubs in English football history.

The club was founded in 1905, taking its name from the adjacent area of Chelsea, and plays its home matches at Stamford Bridge. Domestically, the club has secured six top flight league titles, eight FA Cups, five League Cups, four FA Community Shields, and two Full Members' Cups. Internationally, their honours include two UEFA Champions Leagues, two UEFA Europa Leagues, two UEFA Cup Winners' Cups, two UEFA Super Cups, two FIFA Club World Cups, and one UEFA Conference League.

Chelsea secured their first major trophy, the First Division championship, in 1955, and won their first Premier League title in the 2004–05 season. Their first European honour came in the 1971 UEFA Cup Winners' Cup, followed by a second victory in 1998. They subsequently won the UEFA Champions League in 2012 and 2021, and the UEFA Europa League in 2013 and 2019. After winning the UEFA Conference League in 2025, Chelsea became the first club to win all four main UEFA competitions as well as all UEFA men's competitions. They also won the FIFA Club World Cup in 2021 and 2025, the latter of which was the first iteration of the tournament with 32 teams.

Nicknamed the Blues, the team adopted their distinctive royal blue home strip early in the club's history, moving away from their original light blue shirts. Also associated with the club is their traditional anthem, "Blue Is the Colour". Chelsea contest the West London derby against local rivals Fulham, alongside fierce capital fixtures with Arsenal and Tottenham Hotspur. They also share a prominent and long-standing rivalry with Leeds United.

Founding and early years (1905–1952)

In 1904, English businessman Gus Mears acquired the Stamford Bridge athletics stadium in Fulham with the aim of turning it into a football ground. An offer to lease it to nearby Fulham F.C. was turned down, so Mears opted to found his own club to use the stadium. As there was already a team named Fulham in the borough, the name of the adjacent borough of Chelsea was chosen for the new club; names like Kensington FC, Stamford Bridge FC and London FC were considered. Chelsea F.C. was founded on 10 March 1905 at The Rising Sun pub (now The Butcher's Hook), opposite the present-day main entrance to the ground on Fulham Road, and were elected to the Football League shortly afterwards.

Chelsea won promotion to the First Division in their second season, and yo-yoed between the First and Second Divisions in its early years. The team reached the 1915 FA Cup final, where they lost to Sheffield United at Old Trafford, and finished third in the First Division in 1920, the club's best league campaign to that point. Chelsea had a reputation for signing star players and attracted large crowds. The club had the highest average attendance in English football in ten separate seasons including 1907–08, 1909–10, 1911–12, 1912–13, 1913–14 and 1919–20. They were FA Cup semi-finalists in 1920 and 1932 and remained in the First Division throughout the 1930s, but success eluded the club in the inter-war years.