The fire broke out at Ninho do Urubu, a state-of-the-art training centre that was expanded and opened only two months ago after a £4.8m (R87m) investment.
According to Reuters news agency, the site features accommodation for young players, several pitches, an aquatic park, a gym, a medical centre and a mini stadium.
None of the victims has been identified but players aged between 14 and 17 are known to have used the dormitory.
"The kids from the Flamengo youth team were sleeping there at the time," firefighter Douglas Henaut told Globo News.
Flamengo u17 player says that the fire started in his room with the air conditioning, but he ran away alive. https://t.co/3Ph4tgQqQ0— Aaron Bauer (@ABauer_ATR) February 8, 2019
Some of its most prominent former players include World Cup winners Ronaldinho, Bebeto and Romario, and more recently wonderkid Vincius Jr.