The Kosi river is one of the largest tributaries of the Ganges. Some of its headstreams rise beyond the Nepalese border in Tibet. About 48 kilometres north of the Indian-Nepalese frontier, the Kosi is joined by several major tributaries and breaks southward through the Shivalik Hills at the narrow Chatra Gorge. The river then emerges on the great plain of northern India in Bihar on its way to the Ganges.

Over the last 250 years, the Kosi has shifted its course over 120 kilometres from East to West and the unstable nature of the river is attributed to the heavy silt it carries during the monsoon season.

Kosi is known as the "sorrow of Bihar", as it has caused widespread human suffering in the past due to flooding and very frequent changes in course, when it flows from Nepal to Bihar.

The Kosi river fan located in the northern part of India (in north-east Bihar) is one of the largest alluvial cones built by any river in the world. It is 180 km long and 150 km wide and shows evidence of lateral channel shifting exceeding 120 km during the past 250 years through more than 12 distinct channels.

Kosi, which used to flow near Purnea in the 18th century, now flows west of Saharsa.

Kosi is mentioned in the epic Mahabharata as Kausiki. The river is also the lifeline of the Mithila region, today spread over more than half of Bihar, and parts of adjoining Nepal and forms the basis of legend and folklore of the region.