The Siberian weasel (Mustela sibirica), also known as the kolonok, is a medium-sized species of weasel native to Asia. It is classed as Least Concern for extinction by the IUCN, due to its wide distribution and presumably large numbers.

In form and hunting behaviour, the Siberian weasel represents a transitional form between small mustelids (such as stoats and least weasels) and their larger cousins (minks and polecats). It is a valuable furbearer, particularly for the paint brush industry.

Siberian weasels have long, stretched out bodies with relatively short legs, but are more heavily built than solongois, stoats and least weasels. Their heads are elongated, narrow and relatively small, and their ears are broad at the base, but short. Their tails represent half their body length. Siberian weasels are much larger than stoats and solongois, and almost approach ferrets and minks in size. Adult males are 28–39 cm long, while females reach 25-30.5 cm. The tail in males reaches 15.5–21 cm in length, while that of females reaches 13.3-16.4 cm. Males weigh 650-820 g, while females weigh 360-430 g. Exceptionally large individuals have on rare occasions occurred in the Baraba steppe. The skull is in several respects intermediate in form between that of the stoat and the mink ; it is longer and larger than that of the stoat, but is somewhat more flattened than the mink's.