The 18 year-old is on a two season loan in Dortmund.
"Dortmund was already interested when I was still in Brazil," he said. "That made me very happy at the time because BVB is very much appreciated by many in Brazil. It didn't work out at the time, so I switched to Real Madrid.
"But when the request came again, I didn't have to think twice. Dortmund is a great opportunity for me, it's a big project and it's definitely the right choice."
Reinier admits he's excited to be playing in the same team as Marco Reus.
He added: "When I was younger, I watched a lot of Borussia games in Brazil. I was impressed by the fast game, how they switched. With Reus or Aubameyang and Lewandowski, things always went forward with power. That's why I'm particularly happy that I'm still on the pitch with Reus. That's something very special for me."