Both Chelsea and Dortmund agreed for a contract in November 2018 for the American midfielder whose contract expires in 2020.
The 20-year-old joined Dortmund as a youth player in 2015 and has made over 100 appearances for the club.
And shortly after his move was confirmed, the forward took to Twitter to address the Dortmund fans in a heartfelt open letter.
He wrote, “It is with a heavy heart that I announce that I will be entering my final months at your world-famous Club.
” It feels like only yesterday that I arrived in Dortmund as a raw, excited, very nervous but exceptionally proud 16-year-old from a small American town called Hershey. I had no idea what lay ahead but I soon discovered that as a young black and yellow player, I would be in the best possible hands.
“I would not be where I am today without the Club and their belief in giving young players a chance. It’s been an honour and a dream to pull on the black and yellow in front of the Sudtribune!
“Many, many memorable moments stand out – to have experienced all of them before my 20th birthday still feels, at times, like a dream. I would like to thank all those coaches who I have played under and who gave me these opportunities.
“And to my amazing team-mates throughout the years: You took me in as a young boy, shared your experience with me, helped me to develop and grow into a professional footballer. For this I sincerely thank you.
“To our unbelievable fans: It’s been a privilege to stand with you on the Sudtribune and to experience how you support your heroes. I feel honoured to have been able to make the trip from being supporter, to being supported. You have taken me in as one of your own. Thank you.
“I would also like to thank Mr. Watzke and Mr Zorc. It was very important to me that I handled my situation in as fair and professional manner as possible towards the Club. So I thank them for finding a solution that was a win-win for everyone.
“In summer of 2019, I move on to Chelsea and to new competition, the English Premier League. 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. Looking forward to working with Maurizio Sarri and his technical team.
“But for the next six months, it’s all BVB! Please do not doubt my passion, determination and my 110 per cent commitment to the team right until the last ball is kicked this season. We all feel that this is ‘our year’ and we are determined to achieve the best possible results in all competitions.’
Pulisic has scored 15 goals in 115 games for Dortmund since making his debut as a teenager back in 2016. In 18 appearances this season, Pulisic has netted three times.