Chelsea Football Club is an English professional football club based in Fulham, West London. Founded in 1905, they play their home games at Stamford Bridge. The club competes in the Premier League, the top division of English football. They won their first major honour, the League championship, in 1955. The club won the FA Cup for the first time in 1970, their first European honour, the Cup Winners' Cup, in 1971, and becoming only the third English club to win the Club World Cup in 2022.

Chelsea are one of five clubs to have won all three pre-1999 main European club competitions, and the only club to have won all three major European competitions twice. They are also the only London club to have won the Champions League and the Club World Cup. Domestically, the club has won six league titles, eight FA Cups, five League Cups, and four FA Community Shields. Internationally, they have won the UEFA Champions League, the UEFA Europa League, the UEFA Cup Winners' Cup and the UEFA Super Cup twice each, and one FIFA Club World Cup since their inception. In terms of overall trophies won, it is the fourth-most successful club in English football.

The club has rivalries with neighbouring teams Arsenal and Tottenham Hotspur, and a historic rivalry with Leeds United. In terms of club value, Chelsea are the seventh most valuable football club in the world (as of 2021), worth £2.39 billion ($3.2 billion), and are the eighth highest-earning football club in the world, with earnings of over €493.1 million (as of May 2022).

In 1904, 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 also 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.