The 21-year-old joins Derek McInnes’ squad on the final day of the summer transfer window.
“I am very happy to be here and excited to start,” he told the club website.
“I know it’s a tough league with a lot of duels and intensity, so I will need to work.
“I have a lot of experience now, so I will try to put it into this league.”
Bouanani developed at Lille before moving to Nice in 2022, where he made his senior debut in January 2023.
He later enjoyed a loan spell at Lorient before returning to Nice. The scored against Rangers in the Europa League in November 2024 with a stunning free-kick.
After 70 appearances for Nice, he joined Stuttgart in 2025, making 28 appearances last season.
Internationally, he represented France at youth level before switching allegiance to Algeria.