Thorgan Hazard has been sold by Borussia Monchengladbach to Borussia Dortmund for €25.5 million ($28.5 million).
Hazard signed a five-year contract with Dortmund.
"We are delighted, Thorgan has opted for Borussia Dortmund with his full conviction," BVB sporting director Michael Zorc said. "He's an experienced Bundesliga player and a Belgium international. He will help us with his pace and his nose for goals."
Hazard had been on the books at Chelsea, but left the club for Gladbach in 2014, initially on a loan and then on a permanent transfer. In his five years for the Foals, Hazard contributed 46 goals and a further 44 assists in 182 appearances across all competitions.
"I am thankful for five great years at Gladbach," Hazard said, adding that it was time to make the next step in his career. "I am proud to be able to play for Borussia Dortmund, a top club with incredible fans."