OFFICIAL: Chelsea sign 20-year-old BVB winger Bynoe-Gittens for a fee of £52M
Talks had been progressing in the US, where both teams had been taking part in the Club World Cup and despite Bayern Munich also being interested, the 20-year-old has agreed a long-term deal at Stamford Bridge.
Dortmund confirmed the transfer had been agreed in Florida on Thursday evening.
The west London club have been looking to strengthen the wide areas following the decision not to take up the option to sign Jadon Sancho permanently.
The England U21 international contributed 12 goals and five assists in 48 games in all competitions for Dortmund last season.