Rømø (röm`ö'), island (1992 pop. 808), 39 sq mi (101 sq km), SW Denmark, in the North Sea, one of the North Frisian Islands, connected with Jylland by a causeway. Rømø and Lafolk are the chief towns. A whaling base in the 18th cent., it is now a bathing resort and has fisheries. The island was held by Prussia from 1864 to 1920.