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Weah, the 19-year-old son of former African footballer of the year and current Liberian President George Weah, has signed a five-year contract with the northern French club.
The deal is reportedly worth around 10 million euros ($11.4 million).
Having struggled to break into PSG's star-studded first team, Weah spent last season on loan to Celtic, where he won the Scottish title.
His sale to Lille is widely seen as part of Qatar-owned PSG's attempts to conform with UEFA's Financial Fairplay regulations designed to prevent clubs from overspending.
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PSG have already sold 19-year-old Moussa Diaby to German club Bayer Leverkusen this summer.
Lille finished runners-up to PSG last season.
New York-born Weah, who joined PSG from New York Red Bulls in 2014, has made eight appearances for the US national team, scoring once.
His father made 138 appearances for PSG from 1992 to 1995.