The Kosi River (Hindi: कोसी नदी) or Koshi (Nepali: कोशी नदी)—also Saptakoshi (Nepali: सप्तकोशी) for its seven Himalayan tributaries—is a trans-boundary river flowing through Nepal and India. Some of the rivers of the Koshi system, such as the Arun, the Sun Kosi and the Bhote Koshi, originate in the Tibet Autonomous Region of China. It is one of the largest tributaries of the Ganges.

Along with its tributaries, the river drains 29,400 km² (11,400 sq mi) in China (mainly the upper Arun basin north of the Mount Everest region), 30,700 km² (11,900 sq mi) in Nepal (the eastern third of the country), and 9,200 km² (3,600 sq mi) in India.

The river basin is surrounded by ridges, which separate it from the Yarlung Tsangpo River in the north, the Gandaki in the west and the Mahananda 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 twelve distinct channels with shifting courses due to flooding.Kamlā, Bāghmati (Kareh) and Budhi Gandak are major tributaries of Koshi in India, besides minor tributaries like Bhutahi Balān.

Kosi may refer to: