Green Island is a small volcanic island in the Pacific Ocean about 33 km (21 mi) off the eastern coast of Taiwan. It is 15.092 km2 (5.83 sq mi) at high tide and 17.329 km2 (6.69 sq mi) at low tide, making it the Republic of China's fourth-largest island. The island is administered as Lüdao Township, a rural township of Taitung County and one of the county's two offshore areas (the other being Orchid Island). It is primarily noted for its prisons and penal colonies.