Aouchiche, 18, penned a three-year deal on Monday and will wear the number 17 shirt.
✍️✅ Le jeune milieu de terrain, nouveau numéro 1️⃣7️⃣, est désormais lié à l'#ASSE jusqu'en 2️⃣0️⃣2️⃣3️⃣ !
— AS Saint-Étienne (@ASSEofficiel) July 20, 2020
He was wanted by a host of top European sides after refusing to extend his PSG contract.
But with playing time on his mind, Aouchiche decided to stay in Ligue 1 by signing for ASSE.
After losing Tanguy Kouassi to Bayern, PSG have watched another of their biggest prospects go.
Aouchiche is a France under-17 international and has the option to play for Les Bleus or Algeria at senior level.