The 33-year-old began his career at the Stade Bollaert-Delelis, making 16 senior appearances before moving to Chelsea in 2012, following the path of his brother Eden.
"The return of a former player is a wonderful symbol, especially one who is a true product of La Gaillette,” Sporting Director Jean-Louis Leca told Lens website.
“With Thorgan, we welcome a player with 522 club appearances, 64 European matches, and 47 caps for Belgium. This top-level experience will be a real asset to the team this season, which will see us return to the Champions League.
“Thorgan will bring us his ability to read and accelerate the game, his passing quality, and his leadership. His strong sense of family and his down-to-earth nature brought us together.
“During our discussions, I sensed his attachment to RC Lens and a strong desire to represent the club in the Champions League."
Hazard went on to represent several clubs, including Zulte Waregem, Borussia Monchengladbach, Borussia Dortmund, PSV Eindhoven and RSC Anderlecht.
He spent the last two seasons at Anderlecht, where he enjoyed an impressive campaign, scoring 15 goals and providing 11 assists in 46 appearances.
After his contract expired, Hazard chose not to extend his stay and instead returned to Lens, who will compete in the UEFA Champions League next season.
He becomes the club’s second summer signing after Michael Cuisance and will wear the number nine shirt.