Christian Pulisic says he is privileged to have signed for "legendary" Chelsea in a deal worth €64million.
The 20-year-old United States international will stay at Borussia Dortmund for the rest of the 2018-19 season, contributing to their Bundesliga title push.
Pulisic, who was also linked with Premier League leaders Liverpool, will then switch to Stamford Bridge for the start of the 2019-20 campaign.
"In summer I move on to Chelsea and to a new competition, the English Premier League," Pulisic told Chelsea's official website.
"It's a privilege to have signed for such a legendary club and I look forward to working hard towards being a contributor to their team of world-class players."
Chelsea director Marina Granovskaia hailed Pulisic's potential after the 20-year-old's switch was agreed.
"We are delighted to have signed one of Europe's most sought-after young players," she said in a club statement.
"Christian has shown his quality during a fantastic spell in Germany and at just 20, we believe he has the potential to become an important Chelsea player for many years to come.
"We look forward to welcoming him to Stamford Bridge in the summer and wish him and Dortmund every success for the remainder of the season."