West Ham United Football Club is a professional football club based in Stratford, East London, England. The club competes in the Premier League, the top tier of English football. The club plays at the London Stadium, having moved from their former home, the Boleyn Ground, in 2016.

West Ham United was founded in 1895 as Thames Ironworks and reformed in 1900 as West Ham United. It moved to the Boleyn Ground, which remained its home ground for more than a century, in 1904. The team initially competed in the Southern League and Western League before joining the Football League in 1919. It was promoted to the top flight in 1923, when it was also losing finalist in the first FA Cup final held at Wembley. In 1940, the club won the inaugural Football League War Cup.

West Ham United has won five major honours in its history. Domestically, it has been winner of the FA Cup three times (1964, 1975 and 1980) and runner-up twice (1923 and 2006). In European competitions, the club has reached three major European finals winning the European Cup Winners' Cup in 1965, finishing runner-up in the same competitions in 1976, and winning the second edition of the Conference League in 2023. The club has also won one minor European trophy by winning the Intertoto Cup in 1999. West Ham United is one of eight clubs never to have fallen below the second tier of English football, spending 66 of 98 league seasons in the top flight, up to and including the 2023–24 season. The club's highest league position to date came in 1985–86, when it achieved third place in the then First Division.

Three West Ham players were members of the 1966 World Cup finals-winning England team: captain Bobby Moore and goalscorers Geoff Hurst and Martin Peters. The club has a long-standing rivalry with Millwall, and the fixture has gained notoriety for frequent incidents of football hooliganism. West Ham adopted their claret and sky blue colour scheme in the early 1900s, with the most common iteration of a claret shirt and sky blue sleeves first emerging in 1904.