The 54-year-old says he has had contact from fellow Ligue 1 sides Lyon and Nice as well as Serie A team Napoli.
Galtier joined Lille in December 2017 after having an eight-year run in charge of Saint-Etienne.
"I simply have the deep belief that my time is up here," he told L'Equipe on Tuesday.
"I've made up my mind. I informed my president, Olivier Letang, after listening to him a lot this morning that I was going to leave LOSC.
"This four-year cycle is long enough for a coach. I feel inside that this is the time to leave this beautiful club, this magnificent club."
It was just the club's fourth league title in the professional era and their first since 2010-11, when Eden Hazard was part of a star-studded squad.
Lille's points tally of 83 was the best in their history and they only lost three matches all season, the last of which was in March against Nimes.
Their success was only the second time in nine seasons that PSG have not been crowned champions, with Monaco also having lifted the trophy back in 2016-17.
Galtier, who replaced Marcelo Bielsa when he took over at Lille, saved the club from relegation in that 2017-18 season.
He then led them to a second-place finish in 2018-19 and to fourth last season, which was cut short due to the coronavirus pandemic.
Veteran striker Burak Yilmaz was one of the stars of a Lille squad who upset the odds in 2020-21 and ensured the likes of Mauricio Pochettino, Neymar and Kylian Mbappe missed out on Ligue 1 silverware.