The Onon is a river in Mongolia and Russia. It is 1,032 kilometres (641 mi) long, and has a drainage basin of 96,200 square kilometres (37,100 sq mi).
It originates at the eastern slope of the Khentii Mountains. For 298 km it flows within Mongolia, before entering the Khentei-Daur Highlands region of the Russian Far East flowing along the Stanovik Range. In its lower reaches it flows between the Mogoytuy and the Borshchovochny mountain ranges.
It joins with the Ingoda to form the Shilka.
Its river valley forms the eastern limit of the Khentei-Daur Highlands.
Its main tributaries are the Khurkhyn Gol, Borzya and Unda from the right, and the Agutsa, Kyra and Aga from the left.
The Onon—Shilka—Amur water system is one of the world’s ten longest rivers, measuring 1,032 km + 560 km + 2,874 km.
