The Premier League club paid 64 million euros ($73.2 million) for the 20-year-old, Dortmund said in a statement.

"It has always been Christian's big dream to play in Premier League," Dortmund sporting director Michael Zorc said.

"This certainly has something to do with his American origin and thus we were not able to prolong his contract.

"Because of this background and with his contract running down in 2020, we decided to accept Chelsea's incredibly lucrative offer.