Isla San Ildefonso, is an island in the Gulf of California east of the Baja California Peninsula. The island is uninhabited and is part of the Mulegé Municipality.
Isla San Ildefonso has four species of reptile: Coluber flagellum (coachwhip), Phyllodactylus nocticolus (peninsular leaf-toed gecko), Sceloporus orcutti (granite spiny lizard), and Uta stansburiana (common side-blotched lizard).