The 19-year-old, who has been given the number 32 shirt, will join up with his new team-mates at the start of the pre-season training on 1 July.
Hannover's recently appointed sporting director Dirk Dufner was delighted to have sealed the deal: "We decided to bring Leo Bittencourt to the club because we were convinced of his ability. He has all the ingredients to be an asset to the team," said the 45-year-old.
Head coach Mirko Slomka echoed those sentiments. “I’m very pleased we've been able to reach an agreement with BVB. Leonardo made it clear early on that he wanted to come here and I want to develop his talent in the Bundesliga. The club is acquiring a quick and creative attacking player."
"Want to show what I can do"
Bittencourt expressed his delight at joining a side where he can hope to play more first-team football. "I'm a passionate footballer. The biggest thing for me is to show as often as possible what I can do," said the former Energie Cottbus youngster. "My friend Andre Hoffmann is the best example of how young players can win a place in the Hannover team and that is my goal."
Bittencourt joined Dortmund from Bundesliga 2 side Cottbus in 2012 and made five Bundesliga appearances in the season just ended, scoring once. He has also made one appearance for Germany's U-21 side.