The midfielder had been courted by several top Ligue 1 sides but he ultimately opted to move to the Geoffroy Guichard, putting pen to paper on a four-year contract with Les Verts
The midfielder had been linked with Marseille and Bordeaux but he has now penned a four-year contract with Les Verts.
"I am delighted that the negotiations were conducted in an atmosphere of trust and mutual respect," Saint-Etienne chief executive Roland Romeyer told his club's official website.
"I am also delighted that a player like Romain, coveted by several top teams, has joined our sports project. This transfer shows that we are an attractive club."
The 25-year-old Hamouma, who scored two goals in 31 Ligue 1 appearances last season, joined Caen from Laval in 2010.