Weah, the son of former AS Monaco and AC Milan player and current Liberia president George, joined PSG in March last year and made six appearances for the French champions before being loaned to Scotland's Celtic in January.
He scored four goals in 17 appearances for the Scottish champions last season. Lille CEO Marc Ingla said the French side had beaten several other suitors to get his signature.
"We are very pleased to have signed Timothy Weah; many good clubs were courting him and Timothy chose our proposal," Ingla said in a statement on Lille's website https://www.losc.fr/actualites-foot-lille/weah-est-lillois.
Lille qualified for next season's Champions League after finishing runners-up in France last season and Weah said the chance to play in Europe's elite club competition had helped to make up his mind.
"For me, (Lille) is the best choice to progress and get playing time. It's a young and ambitious team," he said.
"Joining LOSC is a big deal, this team had a great season last year, finishing second is a great performance, playing in the Champions League is also a dream that I will be able to achieve.
"I want to score goals in my new colours and help this club to win games and trophies."
(Reporting by Simon Jennings in Bengaluru; editing by Clare Fallon)
