St Michael's Isle, more commonly referred to as Fort Island, is an island in Malew parish in the Isle of Man, noted for its attractive ruins. It covers an area of 5.14 hectares, is about 400 metres long from west to east, and is connected to the Langness Peninsula, near Derbyhaven, by a narrow causeway. The island itself is made of rocky slate and the soil is very acidic. Nevertheless, it has important communities of maritime plants.