Aouchiche has signed a permanent deal with the Bundesliga 2 side who are currently top of the German second-tier table.
The French midfielder has been recalled from a season-long loan at Scottish side Aberdeen to enable the switch to happen.
His return from Pittodrie frees up an international loan slot, which will enable Simon Adingra to join Monaco on an initial loan, with a view to a permanent transfer in the summer.
That in turn will lead to Sunderland signing winger Nilson Angulo in a permanent transfer from Belgian side Anderlecht.
Aouchiche leaves Sunderland having made 38 senior appearances for the club following his move from Lorient in September 2023.
Sunderland sporting director, Kristjaan Speakman, said: "Loan opportunities are beneficial in the short term, but moving forward, we felt Adil needed a project with a longer-term view.
"He possesses so many qualities and is extremely hard-working, so this opportunity at a prestigious club like FC Schalke 04 is fully deserved. We thank him for his contribution to our club and wish him well for the future."