After starring for Flamengo, Reinier earned a €30 million transfer to Los Blancos in January of 2020, but is yet to feature in the first team for the Spanish champions, instead continuing his development within the Castilla side.
Now 18, he'll join Dortmund until the conclusion of the 2020 campaign.
Real Madrid are keen for the young Brazilian to see more first team football which he is currently unable to get at the Santiago Bernabeu due to non-EU registration rules.
The club is allowed to register three non-EU players for the coming season and those positions are already occupied by Vinicius Jr, Rodrygo, and Eder Militao.
The loan deal between Madrid and Dortmund will be similar to the one involving Achraf Hakimi, who starred for the German side for two years on loan from Los Merengues before eventually earning a move to Serie A with Inter.
"We’ve been keeping tabs on the lad for over two years; in addition to his outstanding technical qualities, he also boasts a strong physical presence.
"Following the long break from competitive football, we’ll give him the necessary time to adapt to professional football in Germany and he can then be an additional option for us going forward."
"I’m happy that I’m now at BVB," he said.
"The club is known for its focus on young players and improving them. I would like to learn a lot and play a lot here, help the team and be successful. I hope that we’ll experience many happy moments together."