A toy dog is a very small dog kept as a pet, as compared to a very small working dog, although both pet and working dogs can be very small. When an all-breed association or kennel club divides dog breeds into groups for purposes of competition, the Toy Group contains most of the smallest dog breeds in the club's registry.

The use of the word "toy" to describe small dogs that belong to a toy breed is redundant and also incorrect, suggesting that the breed comes in different sizes—there is no such thing, for example, as a "toy Chihuahua"; all Chihuahuas are toy dogs. (However, some breeds do come in different sizes, such as Poodles, which come in standard, miniature, and toy varieties).