The 24-year-old made his top-flight debut with Le Tef in the 2008-09 season and scored a total of 15 goals in 138 league appearances for the south-west outfit.
But the left-sided former France Under-21 international switched his allegiance on Monday to Christophe Galtier's St Etienne and the Stade Geoffroy-Guichard for an undisclosed fee.
He told asse.fr: "It makes me really proud to join ASSE and I am determined to have a great season.
"I've spoken to Christophe Galtier on the telephone a lot and the project has me excited.
"I leave Toulouse with a twinge in my heart because they're the club that gave me a break at the highest level, but the page turns and I needed a new challenge to progress."
Tabanou can also play confidently at left-back having adapted to the role to cover injured team-mate Cheikh M'Bengue for much of the 2010-11 season.
Galtier said: "He's a specialist on the left but he can also play on the right and has good experience in Ligue 1.
"He is ready physically but will probably need some time to adapt to the club. I hope Franck enjoys the same success that other players have had and that he flourishes here."