Fluminense Football Club (Brazilian Portuguese: [flumiˈnẽsi futʃiˈbɔw ˈklubi]), known as Fluminense, is a Brazilian sports club best known for its professional football team that competes in the Campeonato Brasileiro Série A, the first tier of Brazilian football and the Campeonato Carioca, the state league of Rio de Janeiro. The club is based in the neighbourhood of Laranjeiras since its foundation, in 1902. Fluminense is the oldest football club in Rio de Janeiro.

The club was founded on 21 July 1902 and Oscar Cox was its first elected president. Fluminense have since been crowned national champions four times, most recently in the 2012 Campeonato Brasileiro Série A; the team have also won the 2007 Copa do Brasil, the 1999 Campeonato Brasileiro Série C and the 1952 Intercontinental Cup. In 1949, Fluminense became the first football club in the world to receive the Olympic Cup, awarded annually by the International Olympic Committee to an institution or association with a record of merit and integrity in actively developing the Olympic Movement. Its best international performances are finishing runner-up in the 2008 Copa Libertadores and 2009 Copa Sudamericana.

Fluminense is a demonym for people indigenous to the state of Rio de Janeiro, in Brazil. Although football was its original endeavour, the club is today an umbrella organisation for several teams in more than 16 different sport activities.

Fluminense's traditional home kit consists of an iconic combination of three colors: garnet, white and green, disposal in vertical stripes, since its adoption, in 1904. Along with white shorts and white socks, an outfit which has been in use since 1920, that's the typical home kit for the Tricolor.

The club holds several long-standing rivalries with other clubs, most notably with Flamengo (Clássico Fla-Flu), as well as with Botafogo (Clássico Vovô) and Vasco da Gama (Clássico dos Gigantes). The Clássico Fla-Flu is widely considered the greatest football derby of Brazil, and host several attendance records, as the two highest attended matches in any football club tier, with almost 200.000 supporters in Maracanã.

The club is the birthplace of the Brazil national football team, which played its first game midst the celebrations of the 12th anniversary of the club. In Fluminense's ground, the Stadium of Laranjeiras, the Canarinhos held their first ever match, scored their first ever goal and lifted their first ever trophy. Until today, the club has contributed the fifth-most players to the national team among all Brazilian clubs.