The Katun (Russian: Катунь) is a river in the Altai Republic and the Altai Krai of Russia. It forms the Ob as it joins the Biya some 19 kilometres southwest of Biysk. The Katun is 688 kilometres long, and its drainage basin covers 60,900 square kilometres. It originates in the Katun glaciers on the southwestern slope of Belukha Mountain. The river freezes up in late November or early December and breaks up in early or mid-April.
