AC Milan have signed the United States international forward for £17m and he has penned a four-year contract with the option for a further year.
Pulisic is the second Chelsea player to join AC Milan this summer after Ruben Loftus-Cheek also moved to the San Siro earlier this month.
Pulisic joined Chelsea from Borussia Dortmund in the summer of 2019.
The 24-year-old made 145 appearances for Chelsea and scored 26 goals.
He leaves Stamford Bridge with winners medals from Chelsea's triumphs in the 2021 Champions League, UEFA Super Cup and FIFA Club World Cup.
A statement from the seven-time European champions read: "AC Milan is happy to announce the signing of Christian Pulisic from Chelsea FC. The American winger has signed a contract with the Rossoneri until 30 June 2027, with an option to extend his stay until 30 June 2028.
"Born in Hershey (USA) on 18 September 1998, Christian came through the Youth Sector in the USA before moving to Borussia Dortmund in 2015. With the Black and Yellows, he made his senior debut in January 2016 and featured in 127 games scoring 19 goals, winning 1 German Cup. In January 2020, he moved to Chelsea, where he featured 145 times, scoring 26 goals and winning 1 Champions League, 1 European Super Cup and 1 Club World Cup.
"He’s been capped 60 times for the USA's American national team, scoring 25 goals for the USMNT and winning the CONCACAF Nations League twice.
"Christian Pulisic has chosen the number 11 shirt."