Tabanou, who has represented France at U21 level 12 times, is believed to have cost around £3.5m, and becomes Swansea’s second signing of the summer, following the capture of forward Andre Ayew from Marseille.
The defender has been on Swansea boss Garry Monk’s target list for some time, and he nearly joined the club in the January transfer window.
Tabanou, 26, began his career at Toulouse, scoring 15 goals in 138 games, before joining St Etienne in 2013.
He played in the Europa League and helped St Etienne to the French Cup semi-finals last season.
“Swansea is a very good Premier League club and I believe it’s the right move forward for me at this stage of my career,” he told the club’s website. “I wanted to come to Swansea because I believe we are the perfect match.
“I was aware of Swansea’s interest in me for a while, but unfortunately it didn’t happen back in January. But I’ve been watching them closely ever since and I like the way they play the game; their whole philosophy on football.
“Once I spoke to the manager and the Chairman my mind was made up because I realised they really wanted me. Again, it all matched.
“I was particularly impressed with the manager because he knew everything about me. I was also touched by the fact that he said I was a top signing for Swansea.”
Tabanou will provide competition for first choice left-back Neil Taylor who signed a new long-term contract earlier this summer.