The 24-year-old was the captain at his former club but has chosen to stay in Ligue 1 rather than compete in the division below, and could prove a bargain at just three million euros.
Ruffier made his debut for the French national team last year and it may well be his desire to play for his country that has seen him remain in France.
The goalkeeper has since revealed that he turned down an approach from Manchester City.
Ruffier said: "On Monday afternoon, my agent received a phone call from Manchester City. However, for me the priority was to play at St-Etienne, because sport is the most important aspect.
"Manchester City doesn't matter to me. I am not at City, I am at St-Etienne and I am here for the sporting aspect. I was only attracted to playing."
He will now compete with Jeremie Janot for the number one spot. Janot has been at the club since 1996, making well over 300 appearances.
Ruffier continued: "Did I consider the presence of Jeremie Janot? No I didn't, I was just looking for a high-level club where I could blossom, that's all.
"With Monaco suffering relegation to Ligue 2, I wanted to leave. To me it was essential to remain at a high level. That's why I chose St-Etienne."