Bittencourt, who holds Brazilian and German citizenship, was born in Germany and has played at the Under-17 and U-19 levels for Germany's youth squads. He has played 15 first-team matches for Energie in the Bundesliga 2 this season.
"He's quick, technically gifted and has already come a long way in terms of development. Leo will fit in perfectly with our squad," Dortmund coach Jurgen Klopp said.
Bittencourt signed a four-year contract with the reigning Bundesliga champions - who are also leading the league this season after 14 matches.