The 22-year-old Netherlands international had been with the Bianconeri since 2019, but leaves after two seasons where they have slipped out of the Scudetto race.
De Ligt becomes the German champions’ first major off-season signing and arrives in light of Robert Lewandowski’s official exit for Barcelona.
“I’m very happy to become a player for this great club,” de Ligt said.
“FC Bayern are the most successful club in Germany, one of the most successful clubs in Europe and in the world. I felt the genuine appreciation from the sporting management, the coach and the board right from the start, which convinced me.
“On top of that, FC Bayern is a brilliantly run club with big aims. I’m very glad that I’m now becoming part of the FC Bayern story.”
De Ligt’s addition will help offset the loss of Niklas Sule on a free transfer to Borussia Dortmund.
Julian Nagelsmann’s side have already signed Sadio Mane from Liverpool and Noussair Mazraoui and Ryan Gravenberch from Ajax this off-season.
Bayern president Herbert Hainer added: “Matthijs de Ligt was a top target for us. With signings like this, the big aims that we have are achievable.
“The future face of our team is increasingly taking shape, and this transfer is an important building block in the overall concept that we’re gradually putting into place. De Ligt should become a mainstay at FC Bayern.”