San Marcos is an island located in the northern portion of Baja California Sur, México. It faces the town of Santa Rosalía on the Gulf of California near Isla Tortuga. The only town on the island is also named San Marcos. The island has a land area of 28.748 km² (11.100 sq mi), the 18th-largest island in Mexico. Its population as of the 2005 census was 425 inhabitants.