Celtic's Janko Saidy in action with Inter Milan's Ever Banega
A number of English Championship sides including Sheffield Wednesday, Reading and Bristol City have been weighing up a move for Janko, but St Etienne have moved quickly to agree a deal with Celtic for the full-back.
Janko spent last season on loan at Barnsley and he helped them consolidate in the Championship in their first season back in the second tier of English football with a 14th spot finish.
Celtic's Saidy Janko (R) in action with St Johnstone's Brian Easton
The 21-year-old, who started last season at Celtic and helped them qualify for the group stages of the Champions League, joined Barnsley in a surprise move on transfer deadline day last summer.
Janko scored on his debut for Barnsley against Wolves, but injuries restricted the attacking full-back to just 14 appearances for the Tykes.
Nevertheless Janko impressed during his time at Oakwell and his performances alerted several Championship sides.
Janko returned to Celtic for pre-season following his loan spell, but his first-team chances at Parkhead were limited next season with Mikael Lustig and Cristian Gamboa ahead of him in the pecking order in Brendan Rodgers' plans.
Saint Etienne have offered the Switzerland Under-21 international a move out of Celtic and the defender is in line to sign a four-year deal with Les Verts.
Celtic's Saidy Janko had been linked with Sheffield Wednesday, Reading and Bristol City.