The Kosi or Koshi is a transboundary river which flows through China, Nepal and India. It drains the northern slopes of the Himalayas in Tibet and the southern slopes in Nepal. From a major confluence of tributaries north of the Chatra Gorge onwards, the Kosi River is also known as the Saptakoshi (Nepali: सप्तकोशी, saptakoshī) for its seven upper tributaries. These include the Tamur River originating from the Kanchenjunga area in the east and Arun River and the Sun Kosi from Tibet. The Sun Koshi's tributaries from east to west are the Dudh Koshi, Likhu Khola, Tamakoshi River, Bhote Koshi and Indravati. The Saptakoshi crosses into northern Bihar, India where it branches into distributaries before joining the Ganges near Kursela in Katihar district. The Kosi is the third-largest tributary of the Ganges by water discharge after the Ghaghara and the Yamuna.

The Kosi is 720 km (450 mi) long and drains an area of about 74,500 km² (28,800 sq mi) in Tibet, Nepal and Bihar. In the past, several authors proposed that the river has shifted its course by more than 133 km (83 mi) from east to west during the last 200 years. But a review of 28 historical maps dating 1760 to 1960 revealed a slight eastward shift for a long duration, and that the shift was random and oscillating in nature.

The river basin is surrounded by ridges which separate the Kosi from the Yarlung Tsangpo River in the north, the Gandaki Rive in the west and the Mahananda River in the east. The river is joined by major tributaries in the Mahabharat Range approximately 48 km (30 mi) north of the Indo-Nepal border. Below the Siwaliks, the river has built up a megafan some 15,000 km² (5,800 sq mi) in extent, breaking into more than 12 distinct channels, all with shifting courses due to flooding. Kamalā and Bagmati Rivers are the major tributaries of Kosi River in India, besides minor tributaries such as Bhutahi Balān.

Its unstable nature has been attributed to the power it can build up as it passes through the steep and narrow Chatra Gorge in Nepal. During the monsoon season, it picks up a heavy silt load, which it redeposits at times, causing it to change its channel. This leads to flooding in India with extreme effects. Fishing is an important enterprise on the river but fishing resources are being depleted and youth are leaving for other areas of work.

The Kosi River catchment covers six geological and climatic belts varying in altitude from above 8,000 m (26,000 ft) to 95 m (312 ft) comprising the Tibetan Plateau, the Himalayas, the Himalayan mid-hill belt, the Mahabharat Range, the Siwalik Hills and the Terai. The Dudh-Koshi sub-basin alone consists of 36 glaciers and 296 glacier lakes. The Kosi River basin borders the Tsangpo River basin in the north, the Mahananda River basin in the east, the Ganges Basin in the south and the Gandaki River basin in the west. The eight tributaries of the basin upstream the Chatra Gorge include from east to west:

The three major tributaries meet at Triveni, from where they are called Sapta Koshi meaning Seven Rivers. After flowing through the Chatra Gorge the Sapta Koshi is controlled by the Koshi Barrage before it drains into the Gangetic plain.

The reason for such a large, deep gorge is that the river is antecedent to the Himalayas, meaning that it had existed before them and has entrenched itself since they started rising.

Peaks located in the basin include Mount Everest, Kangchenjunga, Lhotse, Makalu, Cho Oyu and Shishapangma. The Bagmati river sub-basin forms the south-western portion of the overall Kosi basin.