The 47-year-old led Lens to its first Coupe de France title last season as well as a second-place finish behind Paris Saint-Germain in the league.
He replaces Oliver Glasner, who chose to leave Selhurst Park after nearly two-and-a-half years at the helm.
"It's amazing to be here at Crystal Palace," Sage said after agreeing a three-year contract on Monday. "I am excited by the history of the club and by recent seasons. Oliver Glasner achieved some amazing things and now I have to do the same. That's why we come here with a lot of ambition."
Glasner helped Palace win the FA Cup in 2025 and the Europa Conference League title last month. They are the only major trophies in the south London club's 120-year history.
Aside from his success with Lens, which pushed PSG to the final weeks of the Ligue 1 title race, Sage will have also been an attractive option to Palace because he favours a similar approach — a three-at-the-back formation with wing backs and a sole striker — to the one Glasner deployed.
A former amateur player, Sage spent most of his coaching career in the shadows. He returned to Lyon in the summer of 2023 as director of the club's academy after a stint as an assistant at second-division Red Star.
Just months later, he was thrust into his first proper coaching role. When he took charge at the end of November 2023, Lyon had fired Laurent Blanc and Fabio Grosso, and were bottom of the league with one win from 12 matches.
Lyon finished that season in sixth place and reached the French Cup final, losing 2-1 to PSG.
After Lyon sacked him in January 2025, when sixth in the league, Lens soon picked him up
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