It brings to an end the 27-year-old's four-year stay at Le Mans but Matsui, who is in Japan with the national team ahead of the Kirin Cup later this month, believes it was the right time to move on.
"From my playing experience against them, I know that St Etienne are a side who can decide a game when they have to. They have made me feel attracted to join.
"They always have a full stadium at their home games. I believe it is a joy for a player to playing front of crowds like that but I'd like to be accepted by the supporters first."
St Etienne chief executive Vincent Tong-Cuong added that he believes Matsui will bring something special to the club.
"I think Matsui can play well for us in the UEFA Cup," he said.
"Matsui is a beautiful player, with very good technique, speed and talent to beat his opponents.
"That means he will be loved by our fans who know football well.
"I want him to be a fans favourite in St Etienne and become a successor to [former Japan midfielder] Hidetoshi Nakata."