Adrien Rabiot left Marseille for AC Milan on deadline day, just weeks after brawling with a teammate, the two clubs announced on Monday.
Rabiot has signed a two-year deal with Milan for a reported transfer fee of €10 million.
The France midfielder was frozen out of the Marseille squad and placed on the transfer list by coach Roberto De Zerbi after a verbal and physical altercation with Jonathan Rowe following a defeat to Rennes on 15 August.
English winger Rowe has since joined Italians Bologna and the duo could make their debuts for their respective new clubs against each other when Milan host Bologna on 14 September.
"The club's bodyguards were trying to separate them. Normally they're supposed to protect us from others, not from ourselves," Marseille coach De Zerbi said after the incident.
Rabiot was a key player for Marseille last season, scoring 10 goals in 31 appearances after signing from Juventus.
OM finished second in Ligue 1 behind champions Paris Saint-Germain, Rabiot's former club, to qualify for the Champions League.
Rabiot has played 53 times for the French national team and started the 2022 World Cup final defeat by Argentina.
Marseille also announced two signings on deadline day, with 2018 World Cup winner Benjamin Pavard joining Emerson Palmieri in arriving at the Stade Velodrome.
A source told AFP that versatile defender Pavard had agreed a deal to join Marseille from Inter Milan in the closing minutes of the transfer window.
France defender Pavard has not played club football in his home country since leaving Lille for Stuttgart in 2016.
Brazilian left-back Palmieri also joined from West Ham.
The former Chelsea and Roma player made 113 appearances for the London club but was yet to feature this season.