Borneo, island in the extreme southwestern part of the Pacific Ocean. It is the third largest island in the world, surpassed in size by only Greenland and New Guinea. Borneo is situated southeast of the Malay Peninsula in the Greater Sunda Islands group of the Malay Archipelago. The island is bounded by the South China Sea to the northwest, the Sulu Sea to the northeast, the Celebes Sea to the east, and the Java Sea to the south the latter separating Borneo from the island of Java. The Makassar Strait separates Borneo from the island of Celebes (Sulawesi) to the east