“Jamie is an outstanding player who has given us a great deal of pleasure. He now wants to take the next step in his career at Chelsea, and we wish him all the best and much success.”
Gittens, who joined Dortmund from Manchester City in 2020, has scored 17 goals and made 14 assists in 107 games for the club.
He would not be able to play for the Blues until after the Club World Cup.
Chelsea have already signed Ipswich striker Liam Delap, Brighton forward Joao Pedro, Sporting midfielder Dario Essugo and Strasbourg defender Mamadou Sarr this summer.
Estevao Willian will also join Enzo Maresca’s men after his Club World Cup campaign with Palmeiras. He could face the Blues on Saturday too in their quarter-final meeting with his Brazilian club.
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