Greco Island is a marshland in California's Redwood City. The San Francisco Bay edges the eastern and northern extremities of the island. While the Redwood Creek edges the western and the Westpoint Slough edge the southern side of the island. It is a segment of the Don Edwards San Francisco Bay National Wildlife Refuge. The U.S. Fish and Wildlife Service has the ownership of this island.
It is also a habitat for varied species of flora and fauna, including endangered species like the Saltmarsh harvest mouse, the California clapper rail. Further, it is a haul-out site for the harbor seal.