The 20-year-old will remain at the German club on loan until the end of the season before moving to Stamford Bridge in the summer, Dortmund confirmed on Wednesday:
Pulisic had only 18 months left to run on his Dortmund contract, and club sporting director Michael Zorc explained they could not turn down such a good offer given the circumstances, per the club's website (h/t Goal):
"It was always Christian's dream to play in the Premier League. That certainly has to do with his American background, and as a result we were unable to extend his contract. Against this background, we have decided to accept an extremely lucrative bid by Chelsea, given the low contract maturity."
The attacking midfielder posted a farewell message on his Twitter feed:
Pulisic debuted for Dortmund's senior team back in January 2016 having impressed in the youth ranks.
In 2016-17, he played an increasingly important role, making 29 Bundesliga appearances, while he returned four goals and five assists in the German top flight in 32 appearances in 2017-18.
The current campaign has been less fruitful for the United States international.
He has been afforded just five starts in the Bundesliga as Lucien Favre's side have established themselves as front-runners in the title race.
Pulisic remains one of the most exciting young prospects in Europe, though, and Chelsea have done well to sign him.
He should be a valuable attacking asset as he can operate on either flank or through the middle playing as a No. 10.
But even if the Belgian stays put at Stamford Bridge, Pulisic has the quality to become a first-team star under manager Maurizio Sarri.