The 18-year-old, who joined Real Madrid from Flamengo in January, said: “Borussia Dortmund is known for betting on and for young players.
“I need to be very knowledgeable here, play a lot and the team to succeed. I hope we have many moments of satisfaction.”
Dortmund’s director Michael Zorc added: “Reinier is a player who brings great skill and technical skills.”
Dortmund, who finished in the German Bundesliga last season, beat Austria Vienna 11-2 in Sunday’s friendly with Manchester United striker Jadon Sancho among the goalscorers.
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