Vasco da Gama says a 14-year-old boy trying out for the youth squad died during practice on Thursday.
The teenager, whose name was not immediately released, collapsed about half an hour into morning practice in 30-plus (near 90 F) degree temperatures in high humidity in Rio de Janeiro.
Local media said the teenager had a series of seizures after passing out. He was transported to a hospital by the team coach but reportedly was already dead.
Vasco said he had been practicing with the squad for five days.
Authorities will investigate the death.