Wallace, a Brazil under-20 international, was part of the under-17 team that won the South American championship last year and were fourth at the World Cup at the same level.
He made his senior debut at 17 and helped Fluminense win the Brazilian league championship, the Rio State League and the Guanabara Cup.
He will officially join Chelsea in the January transfer window but will remain with Fluminense until the end of the season.
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