The St. Marys River (named Saint Marys River by the USGS, and sometimes misspelled as St. Mary's River) is a 126-mile-long (203 km) river in the southeastern United States. It is named after the Irish Saint Mary. From near its source in the Okefenokee Swamp, to its mouth at the Atlantic Ocean, it forms a portion of the border between the U.S. states of Georgia and Florida.