The 24-year-old Tunisia international, who initially joined Lille on a season-long loan last summer, has agreed a three-year contract with Les Dogues.
Sliti has made just five appearances since his return from the 2017 African Cup of Nations - where he played all four matches as Tunisia exited the tournament in the quarter-finals.
He has played 16 games for Lille, scoring once - during November's 4-2 home victory against Caen.
Sliti, who began his career with Sedan before moving to Paris FC in 2013, has won 11 caps for his country and scored three goals.
Lille are 11th in the table going into Saturday's fixture against Metz.
French side Lille have exercised their option to sign Naim Sliti on a permanent deal when his loan move from second-tier side Red Star expires this summer.