The 19-year-old joins from the Brazilian side, which permitted him to take part in the Blues' pre-season tour to North America prior to the formalities of the deal being concluded.
None of the terms of the deal were disclosed.
The Brazil U-20 international, a left-footed attacker, said on chelseafc.com: "I am very happy to complete my transfer to Chelsea.
"It is an exciting time for me to join the English Premier League champions and I hope I can become an important player for the club.
"I very much enjoyed my time with the squad during pre-season and I am very grateful to Fluminense for allowing me this opportunity.'"
