Fall, who was playing for Dakar-based side Guediawaye FC, signed a two-year deal with the Ligue 1 team, who terminated the contract of their number two goalkeeper Stephane Ruffier on Monday.
"I am very happy to sign for AS Saint-Etienne as this club has already welcomed great Senegalese players. I am proud to follow in the footsteps of Moustapha Bayal Sall, whom I know and who told me a lot about Saint-Étienne.
"I hope to live up to the expectations of the coach and the managers. Right here, all the conditions are met for me to continue my progress."