The Portugal international watched what turned out to be his final appearance as an AC Milan player from the stands during Friday's win over Venezia, before departing to begin a new chapter in Istanbul. Milan will also retain a percentage of any future sale as part of the agreement.
According to Corriere dello Sport, Leão had two offers on the table: Galatasaray were said to be offering around €10m a season plus bonuses, while Aston Villa's proposal was closer to €7.5m annually.
Victor Osimhen, a close friend of Leão's and already at Galatasaray, is understood to have played a part in the decision, contacting the winger directly to convince him to join forces in Turkey.
The transfer will bring Milan a fixed fee of €43m plus €2m in add-ons, along with 20% of any future resale - a figure below the €49.5m the club paid Lille for the winger back in 2019, reflecting a market in which Milan's asking price had started the summer as high as €50-60m.
Leão leaves the club after 291 appearances, 80 goals and 55 assists, having won a Serie A title and an Italian Super Cup during his time in Milan. Visibly emotional after the Venezia match, he said: "My love for this club will never end. I'd have liked to say goodbye differently. Yes, I cried."
Leão is now expected to travel to Istanbul to complete the move, joining a Galatasaray side already boasting Osimhen up front - the club's second major attacking addition in recent transfer windows after the Nigerian's own arrival.
His departure leaves Aston Villa, who had also been chasing him for much of the closing week of the window and had reportedly offered a loan with an obligation to buy, to complete an alternative deal instead for Paris Saint-Germain winger Ibrahim Mbaye.
Milan, meanwhile, head into the coming weeks having sold their most iconic attacking talent of the past decade, with head coach Rúben Amorim already working without him following back-to-back league wins to open the season.