The Katun River (, ) is a river in the Altai Republic and the Altai Krai of Russia. It forms the Ob River as it joins the Biya River some southwest of Biysk. The Katun River is long, and its drainage basin covers . It originates in the Katun glaciers on the southern slope of Belukha Mountain. The river freezes up in the late November or early December and breaks up in the early or mid-April. The Katun River has a few tributaries: Argut River, Chuya River, Koksa River and Sema River. The river is navigable. The upper reach of the Katun River flows down the distant and sparsely populated area, but a few kilometers downstream near the Kujus village, the coastal population density grows steadily and the area downstream of the Ust-Sema village is the most populated. There are numerous buildings, holiday camps and various guest houses in the pine forest near the village.