The ferret (Mustela putorius furo) is the domesticated form of the European Polecat, a mammal belonging to the weasel genus of the family Mustelidae. They typically have brown, black, white, or mixed fur. They have an average length of 20 inches (51 cm) including a 5 inch (13 cm) tail, weigh about 1.5–4 pounds (0.7–2 kg), and have a natural lifespan of 7 to 10 years. Ferrets are sexually dimorphic predators with males being substantially larger than females.

Several other small, elongated carnivorous mammals belonging to the Mustelidae family also have the word ferret in their common names, including an endangered species, the Black-footed Ferret.

The history of the ferret's domestication is uncertain, like that of most other domestic animals, but it is likely that ferrets have been domesticated for at least 2,500 years. They are still used for hunting rabbits in some parts of the world today, but increasingly they are kept simply as pets.

Being so closely related to polecats, ferrets are quite easily able to hybridize with them, and this has occasionally resulted in feral colonies of polecat-ferret hybrids that have been perceived to have caused damage to native fauna, perhaps most notably in New Zealand. As a result, some parts of the world have imposed restrictions on the keeping of ferrets.