Janko 17 times for Forest last season, but found himself frozen out by Martin O'Neill in the second half of last season.
But the right-back will spend the coming campaign with BSC Young Boys, the champions of his native Switzerland, after agreeing another temporary exit from FC Porto.
And he is expected to replace another former English-based player in Kevin Mbabu, formerly of Newcastle United, who recently joined VFL Wolfsburg.
Janko's departure, as well as that of the on-loan West Ham United right-back, Sam Byram, has left Tendayi Darikwa as Nottingham Forest's only specialist in the position.
It remains to be seen whether the new Forest boss, Sabri Lamouchi, intends to strengthen his options in that department.
The Frenchman recently inherited a squad of 30-plus players from his predecessor, O'Neill.
Would Lamouchi have gotten the best out of Janko at Nottingham Forest?
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