Saint Croix (sānt kroi) [key], island (2010 pop. 50,601), 80 sq mi (207 sq km), the largest of the U.S. Virgin Islands, in the West Indies. Christiansted, on the northeast coast, is the island's leading town. A campus of the Univ. of the Virgin Islands is located at Golden Grove, midway between Christiansted and Frederiksted. Buck Island Reef National Monument is off the island's NE coast.