A statement from the player read: 'I would like to express my heartfelt thanks to everyone at Arsenal for the past four years. The experiences and growth I've achieved here have been truly invaluable.
'A special thanks to the coaches, staff, and team-mates at Hale End. The memories made will always be remembered.
'Thank you once again for this journey. I wish everyone continued success and all the best for the future.'
Chido Obi-Martin, 16, pictured (right) celebrating a goal for Arsenal's U18 side last season
Romano claimed via X (formerly Twitter) that the player had accepted a proposal from United, despite being offered more money to move to Germany.
It was said that Obi-Martin chose United because he was convinced by the ongoing project at the Manchester club.
Obi-Martin joined Arsenal from Kjobenhavns Boldklub in Denmark.
He represented Arsenal's U18 side as a 15-year-old, scoring a hat-trick on his first start.
Obi-Martin will likely now be hoping to impress his new manager, Erik ten Hag, at Man United
Obi-Martin has been capped by Denmark at U17 level, scoring 10 goals.
He is also eligible to represent England and Nigeria.