Galtier, who was appointed in 2017, says he has received job
offers from clubs including Nice and Lyon in France and Serie A's Napoli.
The 54-year-old said he was moving on to avoid "falling
into a routine".
Galtier engineered a remarkable turnaround in Lille's
fortunes after succeeding Marcelo Bielsa midway through the 2017-18 season.
Lille just avoided relegation. They then finished second to
Paris Saint-Germain in his first full season.
Lille were fourth when last season was ended early because
of the coronavirus pandemic, and on Sunday were crowned champions for the first
time since 2011.
On the final day of the 2020/2021 season they beat Angers 2-1 to ensure they finished one point ahead of PSG.