Fluminense Football Club (Brazilian Portuguese: [flumiˈnẽsi ˈfutʃibow klɐb]), known simply as Fluminense or Tricolor, is a Brazilian sports club best known for its professional football team plays 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 Laranjeiras neighbourhood of Rio de Janeiro. Fluminense plays their home games at the Maracanã Stadium, which currently holds up to 78,839 spectators.

The club was founded on 21 July 1902 by the sons of Carioca aristocrats, being led by Oscar Cox, a Brazilian sportsman, in the bairro of Flamengo, a direct contrast between the aristocratic founders and the modest ground it was founded on. Cox was elected as the club's first president. Fluminense have been state champions on 31 occasions, second only to Flamengo with 34. The team have been national champions four times, most recently in 2012 in Campeonato Brasileiro Série A, they won the Copa do Brasil in 2007 and the Campeonato Brasileiro Série C , the third tier of Brazilian football in 1999. The Copa Rio de 1952(or Copa Rio Internacional) and the Copa Libertadores runners up in 2008 and the Copa Sudamericana runners up in 2009 are their best international performances.

Fluminense is a demonym for people who reside in the State of Rio de Janeiro. Although football was the club's original endeavor, the club is today an umbrella organisation for several teams in more than 16 different sport activities.

Fluminense's traditional home kit is dark red-and-green vertical striped shirts, with white shorts, accompanied by white socks; this combination has been used since 1920. Umbro is the current kit manufacturer. It holds several long-standing rivalries with other clubs, most notably with Flamengo (this rivalry is popularly known as Fla-Flu), as well as with Botafogo and Vasco da Gama.

The club has contributed the fifth-most players to Brazil's national football team.