The least weasel, little weasel, common weasel, or simply weasel is the smallest member of the genus Mustela, family Mustelidae and order Carnivora. It is native to Eurasia, North America and North Africa, and has been introduced to New Zealand, Malta, Crete, the Azores, and São Tomé. It is classified as least concern by the IUCN, due to its wide distribution and large population throughout the Northern Hemisphere.

Weasels are mammals of the genus Mustela of the family Mustelidae. The genus Mustela includes the least weasels, polecats, stoats, ferrets, and European mink. Members of this genus are small, active predators, with long and slender bodies and short legs. The family Mustelidae, or mustelids, is often referred to as the "weasel family". In the UK, the term "weasel" usually refers to the smallest species, the least weasel (M. nivalis), the smallest carnivoran species.