The fancy rat is the domesticated form of Rattus norvegicus, the brown rat, and the most common species of rat kept as a pet. The name fancy rat derives from the idea of animal fancy (the promotion of domesticated animals) or the phrase "to fancy" (meaning to like or appreciate). Wild-caught specimens that become docile and are bred for many generations still fall under the fancy type. Fancy rats are used widely in medical research, as their physiology is very similar to that of humans. When used in this field, they are referred to as laboratory rats (lab rats).

Domesticated rats are physiologically and psychologically different from their wild relatives, and typically pose no more of a health risk than other common pets. Random color mutations may occur in the wild, but these are rare. Most wild R. norvegicus are a dark brown color, while fancy rats may be anything from white to cinnamon to blue.

The body structure of domesticated rats differs from that of a wild rat as well. The body of a fancy rat is smaller, with larger ears and a longer tail than that of its undomesticated counterpart. They are generally smaller with sharper facial features as well.

Domesticated rats have a longer lifespan than that of wild rats. Because domesticated rats are protected from predators and have ready access to food, water, shelter, and medical care, their average lifespan is around two to three years, in contrast to wild R. norvegicus which average a lifespan of less than one year.

In fiction, pet brown rats are often depicted as tamed rather than domesticated, akin to when a character befriends a wolf. As tamed pets, they have been portrayed in roles that vary from evil to ambiguous to lovable.

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