Fluminense Football Club (Brazilian Portuguese: [flumiˈnẽsi futʃiˈbɔw ˈklubi]), known as Fluminense or more commonly as Flu, is a Brazilian sports club best known for its professional football team that competes in the Campeonato Brasileiro Serie A, the first tier of Brazilian football, and the Carioca Championship, 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.

Fluminense have 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 and the 1952 Copa Rio. In 1949, Fluminense became the only 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 champions of the 2023 Copa Libertadores, and reaching the 2008 Copa Libertadores finals.

Fluminense is the gentilic given to people born in the state of Rio de Janeiro, in Brazil.

Fluminense's traditional home kit consists of an iconic combination of three colors: burgundy, white, and green, disposed in vertical stripes, since its adoption in 1904. White shorts and white socks, an outfit which has been in use since 1920, complement the home kit for O Tricolor.

The club holds several long-standing rivalries with other clubs, most notably with Flamengo (Clássico Fla-Flu), Botafogo (Clássico Vovô) and Vasco da Gama (Clássico dos Gigantes). The Clássico Fla-Flu in particular is widely considered one of the greatest football derbies in Brazil and South America, having eventually set the world record for the highest attendance in a match between football clubs (over 200.000 spectators in the stadium of Maracanã).

The club is the birthplace of the Brazil national football team, which played its first game amidst the club’s 12th anniversary celebrations. In Fluminense's ground, the Estádio de Laranjeiras, the “Canarinhos” held their first match, scored their first goal and lifted their first trophy. To this day, the club has contributed the fifth-most players to the national team among all Brazilian clubs.