Fluminense Football Club (Brazilian Portuguese: [flumiˈnẽsi futʃiˈbɔw ˈklubi]) is a Brazilian sports club based in the neighbourhood of Laranjeiras, in Rio de Janeiro, being the oldest football club in the state since its foundation in 1902. It 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 word \"fluminense\" is the gentilic given to people born in the state of Rio de Janeiro.

Fluminense have won numerous titles throughout its history, including the Copa Libertadores in 2023 and the Recopa Sudamericana in 2024. The club has also claimed four Brazilian championships (1970, 1984, 2010, and 2012), the Copa do Brasil in 2007, and 33 Rio de Janeiro state titles. Other notable achievements include winning the 1952 Copa Rio — regarded by the club as a world title — and being the first football club to receive the Olympic Cup from the International Olympic Committee, in 1949, for its pioneering role in sports. In 2023, Fluminense also reached the FIFA Club World Cup final, finishing as runners-up to Manchester City.

Fluminense’s first home kit originally featured a shirt split in half vertically, with one side in white and the other in grey. However, due to the difficulty of sourcing these colours — particularly during a trip to England in 1904, where club members sought new kits — the club opted to change its colours. A vote was held, and Fluminense adopted its now-iconic home kit: a vertically striped shirt in burgundy, white, and green, with thinner white pinstripes flanked by broader burgundy and green stripes, typically paired with white shorts and white socks.

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 Fla–Flu in particular is widely considered one of the greatest football derbies in Brazil and South America, having eventually set the still-standing world record for the highest attendance in a match between football clubs (over 200,000 spectators in the stadium of Maracanã).

Fluminense is recognized as the birthplace of the Brazil national football team, which played its first-ever match at the club’s Estádio de Laranjeiras in 1914, during Fluminense’s 12th anniversary celebrations. It was there that Brazil scored its first goal and won its first trophy. To this day, Fluminense ranks among the top contributors to the national team, having provided the fifth most players in Brazil’s history.