Clube de Futebol Os Belenenses, commonly known as Os Belenenses (Portuguese pronunciation: [bɨlɨˈnẽsɨʃ]), is a Portuguese sports club best known for its football team. Founded in 1919, it is one of the oldest Portuguese sports clubs. It is based in the 25,000-seat Estádio do Restelo in Belém, Lisbon, hence the club name, which translates as "The ones from Belém". Among its fanbase, the club is commonly nicknamed O Belém, in reference to the neighborhood; Os Pastéis (The Pastries), in reference to a traditional Portuguese pastry originated in Belém; Azuis (Blues) or Azuis do Restelo (The Blues from Restelo), in reference to the club's color and its home stadium; and A Cruz de Cristo (The Order of Christ Cross), for its emblem, or also "Os Rapazes da Praia" (The Boys of the Beach), a reference to the zone of Belém in the earlier 20th century.

Os Belenenses won the 1945–46 Primeira Liga, making them the first club other than the Big Three to win the league title. Os Belenenses has also won six Championship of Portugal/Portuguese Cup trophies, and is the fifth most decorated team in Portuguese football.

Until 1982, Os Belenenses was one of four teams that had never been relegated from the first division. Nowadays, it is the team with the fourth most seasons in the Primeira Liga as well as the team with the fifth most points in the championship's history.

Os Belenenses was the first Portuguese team with a turf pitch and artificial lighting, and was also the first Portuguese club to participate in the UEFA Europa League.

The main sports of the club are football, handball, basketball, futsal, athletics, and rugby union. The club has won national championships in all these sports, but it remains best known for football, its original activity. In the club's history, Os Belenenses has won more than 10,000 trophies, including the first divisions of football, handball, basketball, rugby, and the Portuguese Cup in football and futsal, among other sports.