The Kahayan River, or Great Dayak River, is the second largest river after the Barito River in Central Kalimantan, a province of Indonesia in Kalimantan – the Indonesian part of the island of Borneo. With a total length of 658 km (409 mi) and with a drainage basin of 15,500 km2 (6,000 sq mi) in South Kalimantan, Indonesia. Mean annual discharge 1,178 m3/s (41,600 cu ft/s). The provincial capital Palangkaraya lies on the river. The main inhabitants are Dayaks, who practice slash-and-burn rice cultivation and pan for gold on the upper reaches. The lower Kayahan flows through a rich and unusual environment of peat swamp forests, which has been severely degraded by an unsuccessful program to convert a large part of the area into rice paddies, compounded by legal and illegal forestry.

Central Kalimantan is a province of Indonesia. It is one of five provinces in Kalimantan, the Indonesian part of Borneo. It is bordered by West Kalimantan to the west, South Kalimantan and East Kalimantan to the east, Java Sea to the south and is separated narrowly from North Kalimantan and Malaysia by East Kalimantan's Mahakam Ulu Regency. Its provincial capital is Palangka Raya and in 2010 its population was over 2.2 million, while the 2020 Census showed a total of almost 2.67 million; the official estimate as at mid 2023 was 2,774,747.