The Belgian Footballer of the Year has agreed on a five-year contract with Dortmund.
“I have already signed a five-year deal with Dortmund,” he told Sporza.
“It is a matter of days as to when it will be announced officially. The clubs must be tying up a few loose ends.”
He added: “Everyone knows what Dortmund has – the fans, the Champions League, and they are German champions. It is the best choice for me. I have been a few times to the [Westfalen] stadion and on every occasion when I entered, I felt something, something that you cannot forget.”