The Italy international moves to City for a fee of £26m and has penned a deal to 2030.
Donnarumma told City's website: "To have signed for Manchester City is such a special and proud moment for me.
“I am joining a squad packed with world-class talent and a team led by the one of the greatest managers in the history of football in Pep Guardiola. This is a club every player in world football would love to join.
“I have admired watching Manchester City for many years - so to now be able to play for the club is a huge honour and a privilege. I cannot wait to meet my new team-mates, the staff and the fans. Playing at the Etihad Stadium will be very, very special for me.
“I am very excited for what lies ahead and can promise that I will give absolutely everything to try and help the club achieve even more success.”
Among the best in his position
City's director of football Hugo Viana also said: “Gianluigi’s pedigree, quality and record speaks for itself, and we are all absolutely thrilled he has joined us here at City.
“He has accumulated a wealth of top-level experience and knows what is required to achieve success on a sustained level.
“From a remarkably young age, he has played at the very top level. That is unusual for a goalkeeper and it means he arrives here as one of the best in the world in his position.
"There is no doubt Gianluigi will bring further quality and depth to our top-class goalkeeping department.
“Everyone here is very excited to welcome Gianluigi to Manchester City and we wish him all the very best during his time here with us.”