The guinea pig (Cavia porcellus), also commonly called the Cavy, is a species of rodent belonging to the family Caviidae and the genus Cavia. Despite their common name, these animals are not pigs, nor do they come from Guinea. They originated in the Andes, and studies based on biochemistry and hybridization suggest they are domesticated descendants of a closely related species of cavy such as Cavia aperea, C. fulgida, or C. tschudii, and therefore do not exist naturally in the wild.

In Western societies, the guinea pig has enjoyed widespread popularity as a household pet since its introduction by European traders in the 16th century. Their docile nature, their responsiveness to handling and feeding, and the relative ease of caring for them, continue to make the guinea pig a popular pet. Organizations devoted to competitive breeding of guinea pigs have been formed worldwide, and many specialized breeds of guinea pig, with varying coat colors and compositions, are cultivated by breeders.

If handled correctly early in their life, guinea pigs become amenable to being picked up and carried, and seldom bite or scratch. They are timid explorers, and rarely attempt to escape from their cages, even when an opportunity presents itself. Guinea pigs who become familiar with their owner will whistle on the owner's approach; they will also learn to whistle in response to the rustling of plastic bags or the opening of refrigerator doors, where their food is most commonly stored.

Domesticated guinea pigs come in many breeds, which have been developed since their introduction to Europe and North America. These varieties vary in hair and color composition. The most common varieties found in pet stores are the English shorthair (also known as the American), which have a short, smooth coat, and the Abyssinian, whose coat is ruffled with cowlicks, or rosettes. Also popular among breeders are the Peruvian and the Sheltie (or Silkie), both straight longhair breeds, and the Texel, a curly longhair.