The 25-year-old, previously a member of the Ferrari Academy, will share the role with Italian Antonio Giovinazzi, who also has World Endurance Championship commitments for the Italian manufacturer.
China's first F1 driver raced for three seasons with Sauber, starting 68 grands prix and scoring 16 points with two fastest laps.
A reserve seat opened up at Ferrari after Britain's Oliver Bearman, who competed in three races last year as a stand-in, moved to Ferrari-powered Haas.
Zhou also has links to the new General Motors-backed Cadillac team planning to enter Formula 1 in 2026.
Graeme Lowdon, who has been involved in Zhou's management, is principal of that team and they will use Ferrari engines and gearboxes until General Motors can provide their own.