Through much of its European range, the Least Weasel overlaps with the somewhat larger but otherwise similar Stoat.
Like all weasels, the Least Weasel is a slender animal with a long tail and short legs, enabling it to follow its prey—mostly small rodents—into their burrows. They also kill hares. Its fur is reddish-ginger, brighter than that of most other weasels, with white belly fur, which is not always visible during the summer months. In the northern parts of its range it moults to pure white in winter, as camouflage against the snow, which is why it goes by the name of Snow Weasel in some northern regions, and is called Snow Mouse in Norway and Sweden. It is rarely more than 23 centimeters (9 in) long. Although most active at night, weasels are sometimes seen during the day.
Least Weasels are highly solitary, and even mating does not occur without a fight. Females can breed several times in a year when food is plentiful. Perhaps because of their small size, Least Weasels have an even greater reputation for ferocity than other weasels, and there are many references to them in the popular cultures of different countries.
Traditional Inuit lore held the Least Weasel in great respect because of its pugnacious nature, and the capture of one was regarded as an omen of good luck. In classical and medieval European mythology, it is sometimes said that the only thing which can kill a basilisk is a weasel (by which is meant Mustela nivalis), though it would be killed in the conflict as well. The earliest record of this claim is in Pliny's Naturalis Historia, book 8, par. 33. It was repeated by Isidore of Seville in his Etymologiae, and subsequently by many medieval bestiarists.