"#Turkey calling Mustafa Kapı is cleared for landing at #LOSC #Hoşgeldiniz," Lille said on Twitter.
Kapi is a "young and extremely promising player" of his generation, the French club said in a statement.
The 18-year-old who began his football career in Turkey's Denizlispor Youth team was transferred to Galatasaray in 2014.
Kapi scored three goals in 37 games for Galatasaray.
With the Lions, he won two Turkish Super Lig titles, one Turkish Cup and one Turkish Super Cup.
Kapi became the fourth Turkish player to play for in Lille with Zeki Celik, Yusuf Yazici and Burak Yilmaz.
