With the completion of the deal by June 10, Belingham can play with Dortmund at the club's World Cup, which starts on Sunday.
The opening game Dortmund is opposed to the Brazilian Fluminense team on June 17 at 17:00 GMT.
Bellingham has been named in the England squad for the U-21 European Championship, which will be held in Slovakia from 11-28 June.
But coach Lee Carsley said that he would launch players so that they can compete in the club's World Cup in the United States.
Bellingham continues the footsteps of the older brother Jude by moving to the German league, where he spent the England midfielder – who also came through the Birmingham city system – three years with Dortmund before joining Real Madrid in 2023.
He added in his farewell to Sunderland fans: “I hope that I have made you proud along the way, and in return, the player who reached the highlands I find in the day.
“I will love you and remember you with pride in the rest of my life.”
Dortmund, who won eight titles in the German League, won the Champions League qualification through the fourth place in the German League last season.
