The Dutch center half has left the Bavarian giants after just two years in the Bundesliga.
De Ligt, who was previously at Juventus and Ajax, is reunited with old coach Erik ten Hag.
De Ligt wrote on Instagram : “Dear Bayern fans, I want to thank each and everyone of you for all your love and support. I have spent two amazing years playing for this beautiful club.
“It has been a true honor to have played for a club like Bayern Munich. We have achieved unforgettable moments together. Winning the title as well as several amazing CL- nights. These memories will stay with me forever.
“This is why leaving this club feels bittersweet. I will always remember the love and support I have received from the players, the staff members and from the fans. The recent petition signed by so many of you has touched me deeply.
“Thank you all for the two amazing years. I wish FC Bayern good luck winning many more titles, like Bayern has always done. Mia San Mia.”