The 2021 season is Clube de Regatas do Flamengo's 126th year of existence, their 110th football season, and their 51st in the Campeonato Brasileiro Série A, having never been relegated from the top division.

In addition to the 2021 Campeonato Brasileiro Série A, Flamengo will also compete in the CONMEBOL Copa Libertadores, the Copa do Brasil, and the Campeonato Carioca, the top tier of Rio de Janeiro's state football league and Supercopa do Brasil.

Supplier: Adidas / Sponsor: Banco BRB / Back of the shirt: Mercado Livre / Lower back: TotalEnergies / Shoulder: Sportsbet.io / Sleeves: Havan / Numbers: TIM / Shorts: ABC da Construção / Socks: MOSS Due to the reorganization of the Brazilian calendar caused by the COVID-19 pandemic, the 2021 season started just a week after the end of the 2020 season, making any kind of pre-season or friendly scheduling impossible.

On 24 March 2021 the Brazilian Football Confederation announced the 2021 Campeonato Brasileiro Série A schedule with Flamengo opening the league against Palmeiras at Maracanã Stadium.

Source: TheFinalBall Players in italics have left the club before the end of the season.