Wallace, who was part of the Brazil squad that won the South American U17 championship last year, will remain on loan with their domestic champions until the end of the season.
The attacking full-back made his senior debut earlier this year and has played 18 times in the league for the first team, helping them win the Rio State League and the Guanabara Cup as well as the Brazilian league.
Wallace also helped his country achieve a fourth-place finish at the World Cup in Mexico and is now part of Brazil's U20 side.
He is the third player to move between Chelsea and Fluminense in recent years, following the transfer of Deco and Juliano Belletti to the South American side in summer 2010.
David Luiz, Ramires, Oscar and Lucas Piazon, who was part of the same Brazil U17 World Cup squad, complete the contingent of Brazilian players currently at Stamford Bridge.