The 17-year-old midfielder has been courted by some of Europe’s biggest clubs after enjoying a breakthrough season at the Saint Andrews Stadium in the 2019/2020 season.
Manchester United were Bellingham’s biggest admirers and he was even given a tour of their training facilities by Sir Alex Ferguson back in March.
Dortmund have however stolen a march on the Old Trafford outfit, using the development of Jadon Sancho as an example to get Bellingham’s signature.
Bellingham will play his final game for Birmingham against Derby County in the championship on Wednesday before leaving for Germany.
He has made 43 appearances in all competitions for Birmingham this season, scoring four goals.
"I can't thank Birmingham enough for what they have done for me, not just this past year but since the age of seven,” Bellingham said after completing the move.
"Of course I wish everyone associated with the club nothing but the best. Once a blue always a blue.
"However, I'm incredibly excited to have joined one of Europe's biggest clubs. The direction they are heading in and how much they help to improve young players made it an easy decision for myself and my family.
"I can't wait to perform in front of all the fans when they are back inside one of the most beautiful stadiums in the world."
Dortmund sporting director Michael Zorc said: “He has enormous potential that we share want to develop with him in the coming years.
“He already has a surprising amount of quality in his own possession as well as against the ball, and on top of that, the player also has a strong mentality.
“We immediately see Jude as a reinforcement for our professional squad, but of course we will also give him the time to get used to the higher level.”
