The defender spent last season on loan at the French club and has now made a permanent move to Ligue One.
Signed from Rennes in 2013 for £2.1 million, Theophile-Catherine was an ever present for Cardiff City in their debut Premier League campaign, making 28 appearances, but returned to France following the Bluebirds’ relegation.
The 25-year old has enjoyed his second spell in France becoming a regular for Saint Etienne, featuring 42 times this past season.
Speaking on the club’s official site, Saint Etienne president Roland Romeyer had this to say:
"We're very pleased to have reached an agreement with Cardiff."
"Given the standard of his performances, the option to buy was a priority for us and his versatility is a valuable asset to the team."
It’s a hit on what was paid for him two years ago but represents good value for a player deemed surplus to requirements at the Cardiff City Stadium and the fee will go a long way towards Cardiff’s bid to strengthen before the new campaign.
The Bluebirds have been linked primarily with attacking targets this summer, with Welshman Hal Robson-Kanu chief amongst them, as they seek to add firepower amidst the threat of striker Joe Mason leaving the club.