Financial details were not disclosed but British media reported that the transfer fee for the 27-year-old was around €42 million (around £36 million).
Nkunku moved to Chelsea from RB Leipzig in June 2023 on a six-year deal, having shone during his time in Germany. However, the France international struggled for consistent game time at Stamford Bridge. In 62 appearances for Chelsea, he scored 18 goals.
He won the Conference League and Club World Cup with the Premier League side last season.
Nkunku joins Ardon Jashari, Samuele Ricci and Pervis Estupinan as new additions at Milan, who finished eighth in Serie A last season.
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