A backyard breeder is an amateur animal breeder whose breeding is considered substandard, with little or misguided effort towards ethical, selective breeding. Unlike puppy mills and other animal mill operations, backyard breeders breed on a small scale, usually at home with their own pets (hence the "backyard" description), and may be motivated by things such as monetary profit, curiosity, to gain new pets and/or working animals, or to show children "the miracle of birth".

A backyard breeder is often a substandard breeder of dogs, cats, rats, birds, horses and/or any other kind of domestic companion animal, and the term is used in this sense by the Animal Welfare community, The American Society for the Prevention of Cruelty to Animals (ASPCA), larger established breeders and breed clubs in contrast to the more positive terms "reputable breeder" and "responsible breeder" that describes operations that use responsible animal husbandry and animal breeding methodologies and practices. The ASPCA's definition of a responsible breeder includes one who is mindful to breed for health and function, screens for known heritable deficiencies, breeds physically sound and behaviorally stable dogs and cats, and avoids inbreeding.

Backyard breeding implies either or both of home breeding for non-commercial reasons or a for-profit small commercial operation that does not adhere to good breeding, care and sale practices. Backyard breeders sell the offspring of their animals either to pet stores, directly to pet buyers in person or to pet buyers over the internet. Larger commercial operations of a similar type that breed dogs and/or cats are usually termed a puppy mill (especially in North America) or puppy farm for dogs and a cat mill for cats. However, as large kennels usually require licensing, many puppy mills are licensed with the USDA.

Many small breeders resent the term and its use. Some do so because they assert that they run small but high-quality "boutique" operations. Others argue that their unregistered animals are desirable as companion animals and economical for ordinary people to purchase. Many times backyard breeders choose profit over animal welfare, their animals typically do not receive proper veterinary care. Animals may seem healthy at first but later show issues like congenital eye and hip defects, parasites or even the deadly Parvovirus. The result of this poor care results in unwanted animals.

It has been suggested that backyard breeding continues because there is a market for cats and dogs, especially for those who have been denied access to pet stocks from shelters, rescues and other agencies that may have very stringent adoption or purchase requirements, such as background checks and home visits.