Covering an area of roughly 290,000 square miles, Borneo is the third-largest island in the world. Located southeast of the Malay Peninsula and southwest of the Philippines , Borneo is primarily mountainous. It is divided into four regions: Kalimantan belongs to Indonesia ; Sabah and Sarawak are part of Malaysia ; a small remaining region comprises the sultanate of Brunei . Kalimantan, which is part of the Republic of Indonesia , forms the largest part of Borneo . Sabah and Sarawak are multi-cultural states. Brunei Darussalam is an independent country. Many interesting cultural festivals and celebrations take place in Borneo throughout the year. Two of the most popular parks in Borneo are Mulu National Park in Sarawak, and Mount Kinabalu in Sabah . Borneo 's flora is among the most diverse in the world. It has nearly 11,000 species of flowering plants, about a third of which are indigenous.