Omenainen is a state-owned uninhabited island in the municipality of Nagu, Finland, in the central Archipelago Sea. The size of the island is about seven hectares and its highest point is about 22 meters high. Trees grow there, and the beaches are rugged and inaccessible. The island belongs to the nationally valuable landscape area of Airisto–Seili, and the Natura 2000 area to the Själö archipelago.