The USA international, who had been linked with both Liverpool and Arsenal, will remain on loan with the Bundesliga club until the end of the 2018-19 season.
At the age of 20, the attacking midfielder, who is capable of playing on either wing already boast the experience of 23 senior national team caps, in which he's netted nine goals.
And at club level with Dortmund, Pulisic has amassed 115 first-team appearances, scoring 15 goals and registering 24 assists, since making his debut in 2016.
With only one year left on his current contract, the Bundesliga leaders conceded that offer from the Blues was simply too good to refuse.
"It was always Christian’s dream to play in the Premier League," said BVB sporting director Michael Zorc.
"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."