The Rajang River is a river in Sarawak, Malaysia. The river is located in northwest of Borneo and it originates in the Iran Mountains. The river flows approximately 563 km to the South China Sea, making it the longest river in Malaysia. The upper part of Rajang River is also known as Batang Balui by the Orang Ulu. Malaysia's largest and tallest (160m) hydro electric project, Bakun Hydro Electric Dam Project, is located on Batang Balui narrow Bakun Fall.

The biggest town on the river is Sibu, some 60 kilometers upriver from the mouth of the Rajang. The next major settlement upriver is Kanowit built on the mouth of Kanowit River at the bank of Rajang River, approximately 174 kilometers from the coast of South China Sea and about an hour boat ride from Sibu, followed by Kapit as the last major town on the river.

Environmental History: Tributary of the prehistoric Sunda River

During the Pleistocene epoch or Mesolithic period about 10,000 years ago, there was a 5 degrees Celsius drop in the global temperature. At mountaintops, rainfall as snow and accumulated as huge icy sheets (including Mount Kinabalu), thus making a break in the global hydrological cycle. Due to lack of water discharge into the sea, there was a 120-meter drop of sea levels from the present time. The South China Sea dried up, exposing the Sunda Shelf and previous deep trenches became huge ancient rivers called the North Sunda River.

Asian Mainland, Malay Peninsular, Sumatra and Java became connected to Borneo via the landbridge of exposed Sunda Shelf. The North Sunda River provided vital connection to Mekong River in Vietnam and Chao Phraya River in Thailand to the north, Baram and Rajang rivers in Sarawak to the east and Pahang River and Rompin River to the west of the massive land mass. Freshwater catfishes from those rivers migrated and mated to exchange their genetic materials about 10,000 years ago. Thus, after the Holocene, when the temperature increased, the landbridges and Sunda River were inudated and the catfish populations were isolated. However, their genetic motives are still in the DNA as an evidence of the previous connections of Rajang River to other isolated rivers in Indochina and Borneo.

cs:Rajang ms:Sungai Rajang vi:Sông Rajang tr:Rejang Nehri