The Rajang River is a river in Sarawak, northwestern Borneo, Malaysia. The river originates in the Iran Mountains, flows through Kapit, and then towards the South China Sea. At approximately 565 km long (351 mi), the river is the seventh-longest in Borneo and the longest in Malaysia.

Sarawak is a state of Malaysia. The largest among the 13 states, with an area almost equal to that of Peninsular Malaysia, Sarawak is located in the region of East Malaysia in northwest Borneo, and is bordered by the Malaysian state of Sabah to the northeast, Kalimantan to the south, and Brunei in the north. The state capital, Kuching, is the largest city in Sarawak, the economic centre of the state, and the seat of the Sarawak state government. Other cities and towns in Sarawak include Miri, Sibu, and Bintulu. As of the 2020 Malaysia census, the population of Sarawak was 2.453 million. Sarawak has an equatorial climate with tropical rainforests and abundant animal and plant species. It has several prominent cave systems at Gunung Mulu National Park. Rajang River is the longest river in Malaysia; Bakun Dam, one of the largest dams in Southeast Asia, is located on one of its tributaries, the Balui River. Mount Murud is the highest point in the state. Sarawak is the only state of Malaysia with a Christian majority.