The fancy rat (Rattus norvegicus domestica) 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 use of the adjective fancy for a hobby, also seen in "animal fancy", a hobby involving the appreciation, promotion, or breeding of pet or domestic animals. The offspring of wild-caught specimens, having become docile after having been bred for many generations, fall under the fancy type.

Fancy rats were originally targets for blood sport in 18th- and 19th-century Europe. Later bred as pets, they now come in a wide variety of coat colors and patterns, and are bred and raised by several rat enthusiast groups around the world. They are sold in pet stores and by breeders. Fancy rats are generally easy to care for, and are quite affordable, even compared to other small pets; this is one of their biggest draws. Additionally, they are quite independent, affectionate, loyal and easily trained. They are considered more intelligent than other domesticated rodents. Healthy fancy rats typically live 2 to 3 years, but are capable of living a year or so longer.

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. For example, domesticated brown rats are not considered a disease threat, although exposure to wild rat populations could introduce pathogens like the bacteria Streptobacillus moniliformis into the home. Fancy rats have different health risks from their wild counterparts, and thus are unlikely to succumb to the same illnesses as wild rats.