The Karimata Strait is a wide strait that connects the South China Sea to the Java Sea, separating the Indonesian islands of Belitung to the west and Borneo (Kalimantan) to the east. It is the widest strait between the South China Sea and the Java Sea, but its numerous islands and reefs reduce its navigability. Its weather and current is influenced by the annual southeast and northwest monsoon.

The South China Sea is a marginal sea of the Western Pacific Ocean. It is bounded in the north by South China, in the west by the Indochinese Peninsula, in the east by the islands of Taiwan and northwestern Philippines, and in the south by the Indonesian islands of Borneo, eastern Sumatra and the Bangka Belitung Islands, encompassing an area of around 3,500,000 km2 (1,400,000 sq mi). It communicates with the East China Sea via the Taiwan Strait, the Philippine Sea via the Luzon Strait, the Sulu Sea via the straits around Palawan, and the Java Sea via the Karimata and Bangka Straits. The Gulf of Thailand and the Gulf of Tonkin are part of the South China Sea.