Weasel, a small, aggressive, fur-bearing animal found in many parts of the Northern Hemisphere. It is related to the mink and ferret. The weasel has a small, flat, triangular head with short, rounded ears. It has short legs and a long, slender neck. Weasels range in length from 6 to 24 inches (15 to 60 cm), including the tail. The animal has a strong musky odor. The fur is brown above and white or yellowish below. In cold regions, the fur turns white in winter, though in some species the tip of the tail remains dark. A white weasel is sometimes called an ermine.

The weasel is swift and energetic. If it feels threatened, it may attack animals many times its size—including humans. Animals that prey on weasels include hawks, owls, foxes, and rattlesnakes. Weasels feed chiefly on rabbits, shrews, and various rodents. In many instances weasels kill more animals than they can eat. Weasels hunt chiefly at night.

Weasels commonly nest in the burrows of animals they have killed, or in hollows under rocks or trees. The females ordinarily bear four to six young, which they rear without assistance from the males.

Three species live in North America: the short-tailed weasel, Mustela erminea ; the long-tailed weasel, M. frenata; and the least, or common, weasel, M. rixosa, or M. nivalis. The weasel family is Mustelidae.