The Swiss full-back only signed for the Hoops from Manchester United two years ago, but he found him surplus to requirements by Brendan Rodgers last term and was sent out on loan to Barnsley.
But he failed to really impress during his time in the English Championship and has been unable to break his way back into the Celtic first team during pre-season.
So Janko is now a St Etienne player, signing a four-year deal with the Ligue 1 club.
After completing the move, Janko took to Instagram to say thanks to Celtic for the experience:
In his time at the Hoops, Janko made 20 appearances and played an important role in their run to the Champions League group stages last season.
Mikael Lustig is Celtic’s first choice right back while Cristian Gamboa was added in January and youngster Tony Ralston is breaking through this summer so Rodgers has plenty of cover at right back for the coming campaign.