The 32-year-old spent three-and-a-half years at the Emirates but found first-team chances in the Premier League hard to come by and this season he had been restricted to just seven appearances in cup competitions.
Earlier today it was reported that the Frenchman had asked the Gunners to terminate his contract 18 months early in order to complete his move to France and now Saint-Etienne have confirmed the deal.
"I am very happy and proud to join AS Saint-Etienne," he told the club's website. "I had good discussions with coach Jean-Louis Gasset. I felt that he really wanted me and our exchanges convinced me to come here.
"I want to have fun on the field and help the team of Saint-Etienne to find a better place in the league. Knowing several players like Loic Perrin, Remy Cabella, Yann M'Vila and Stephane Ruffier will facilitate my integration.
"The club has made a good recruitment and we can achieve great things in this second half of the season. I can not wait to play Geoffroy-Guichard where there is a wonderful atmosphere."
Debuchy joins a Saint-Etienne side that are 14th in the Ligue 1 table and two points above the dropzone.