High Quality Content by WIKIPEDIA articles! Sumatra (Indonesian: Sumatera), is an island in western Indonesia, westernmost of the Sunda Islands. It is the largest island entirely in Indonesia (two larger islands, Borneo and New Guinea, are shared between Indonesia and other countries) and the sixth largest island in the world at 473,481 km2 with a population of 50,365,538. Its biggest city is Medan with a population of 2,109,330.