Financial details were not disclosed but British media reported that the Italian club had paid Chelsea 20 million euros (£17m) plus add-ons for the player.
Pulisic has spent four seasons at Chelsea, making 145 appearances, scoring 26 goals and providing 21 assists.
The 24-year-old helped Chelsea win the Champions League, UEFA Super Cup and Club World Cup in 2021.
Pulisic joins a long list of players who have left Chelsea after they finished 12th in the Premier League last season, their lowest finish in nearly three decades.
He becomes the second player to leave the west London club for Milan after midfielder Ruben Loftus-Cheek.
Milan came fourth in Serie A last term to qualify for the Champions League.
Pulisic is Milan manager Stefano Pioli's fourth close-season signing after back-up goalkeeper Marco Sportiello from Atalanta, teenage forward Luka Romero from Lazio and Loftus-Cheek.
