Hammer Island is a small island in the Sacramento–San Joaquin River Delta, in Contra Costa County, California, originally named Santos Island, it was changed to Hammer Island after Hans Hammer purchased the island from Mr. Santos in 1939. Its coordinates are 37°48′59″N 121°33′25″W / 37.81639°N 121.55694°W / 37.81639; -121.55694 (Hammer Island),; the United States Geological Survey measured its elevation as 0 ft (0 m) in 1981, and it appears on a 1978 USGS map.