Dembele was highly-rated in the Celtic academy when he was coming through and made his debut for the club when he was only 16, with his senior debut in 2019.
However, he failed to carve out a niche for himself in the first-team squad and made only nine more appearances before his contract ran out this summer.
Brest have confirmed his arrival with Celtic pocketing a training compensation fee from the deal to take Dembele to France.
The 19-year-old revealed that he saw potential in the project at Brest and is hoping to play more first-team football with the French club.
He told the club’s official site: “I have come to Brest because I saw the project presented by Greg Lorenzi was good.
“It is something that suits me and I think I can have some playing time here.
“It was a great opportunity that presented itself to me and it’s interested me.”
Dembele was just 13 when he featured for the first time in Celtic’s Under-20 side in 2016.
He has signed a four-year contract with Brest and will hope to kick on in his career in France.
