The French midfielder has spent the first half of the season on loan at Scottish Premier League side Aberdeen, making 30 senior appearances in all competitions and scoring three goals.
His loan was due to run to the end of the season, but it featured a recall clause that enables Sunderland to bring his stay at Pittodrie to a premature end.
Talks with Aberdeen took place over the weekend which resulted in Aouchiche leaving Scotland and heading to Germany to undergo a medical and finalise personal terms with Schalke.
READ MORE:
Why Sunderland called off transfer talks with Liverpool over Lutsharel Geertruida
Reaction as Anthony Patterson completes Sunderland loan exit
The 23-year-old is understood to have enjoyed his time with Aberdeen, but regards a permanent move to Schalke as an excellent opportunity to further kick-start his career.
The Gelsenkirchen-based club are currently top of Germany's Bundesliga 2 table as they push for a return to the top-flight.
Aouchiche leaves Sunderland having made 38 senior appearances for the Black Cats since joining the club Lorient in the summer of 2023.
His most recent outing in a Sunderland shirt came in an FA Cup third-round defeat to Stoke City in January 2025 as he spent the second half of last season on loan at Portsmouth.