The 22-year-old had completed a £12.5 million move from Goodison Park to the French top-flight side.
The move was imminent following his inability to secure a work permit due to limited playing time on the international scene.
After changing their manager thrice, they were eliminated from the Champions League, the French Cup and narrowly escaped relegation from Ligue 1.
Onyekuru is expected to make his debut when the Monegasques face Metz on Saturday. Jardim’s men made a losing start to the season at the weekend, losing 3-0 at home to Lyon.