The Onon gol (, ) is a river in Mongolia and Russia of length 818 km and watershed 94,010 km². It originates at the Eastern slope of the Khentii Mountains. For 298 km it flows within Mongolia. Its confluence with Ingoda River produces Shilka River.

The upper Onon is one of the areas that are claimed to be the place where Genghis Khan was born and grew up.

The flow Onon—Shilka—Amur produces one of the world's ten longest rivers (818km + 560km + 2,874km).

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