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.

Property developers Michael and John Taggart are offering a settlement where they would repay less than 1p for every pound they owe.
That arrangement, known as an  Individual Voluntary Arrangement (IVA), would allow them to avoid bankruptcy.
The brothers' house-building business collapsed in 2008. Their creditors will vote on the IVA proposal next month.
In December 2015, the brothers lost a long-running legal battle with the Ulster Bank.

Bradley Lowery, from Blackhall Colliery near Hartlepool, was diagnosed with neuroblastoma in 2013.
More than £700,000 was raised to pay for treatment in the USA, including a donation of £200,000 from Everton.
However, last week his mother Gemma revealed his cancer had grown and treatment would only give him more time.
On Tuesday it was revealed that Bradley had been sent more than 11,000 Christmas cards from well-wishers.
A campaign to encourage people to send him the cards had been organised by an Everton fan.
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.
[NEXT_CONCEPT]
Two County Londonderry brothers facing bankruptcy owe their creditors up to £213m, the High Court has been told.
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A five-year-old boy with terminal cancer has appeared as a mascot at the Sunderland v Chelsea game.