The fancy rat or pet rat is a domesticated breed of the brown rat (Rattus norvegicus) or, more rarely, of the black rat (R. rattus). The name "fancy rat" has nothing to do with the "fanciness" of their appearance but derives from the meaning of "to fancy". Thus, one who keeps pet rats is said to be involved in "rat fancy."

Specially bred as pets at least since the late 19th century, the rats are sociable, intelligent animals and can be trained to use a litter box, come when called, and perform a variety of tricks. Pet rats behave differently than their wild relatives depending on how many generations they have been removed, and, when acquired from reliable sources, they do not pose any more health risk than other, more common pets.

Pet rats live about 2 to 3.5 years, although the oldest rat on record, a lab rat called Rodney, reached a purported age of 7 years and 4 months, according to the 1995 Guinness Book of Records. Bucks (male rats) reach an average weight of 400 g to 800 g, while does (female rats) range from 250 g to 450 g. Female rats tend to be more active than male rats.