A mongrel, mutt, or mixed-breed dog is a dog that does not belong to one officially recognized breed, including those that result from intentional breeding. Although the term mixed-breed dog is sometimes preferred, many mongrels have no known purebred ancestors.

Crossbreed dogs, and "designer dogs", while also a mix of breeds, differ from mongrels in being intentionally bred. At other times, the word mongrel has been applied to informally purpose-bred dogs such as curs, which were created at least in part from mongrels, especially if the breed is not officially recognized.

Although mongrels are viewed as of less commercial value than intentionally bred dogs, they are thought to be less susceptible to genetic health problems associated with inbreeding (based on the theory of heterosis), and have enthusiasts and defenders who prefer them to intentionally bred dogs.

Estimates place the prevalence of mongrels at 150 million animals worldwide.

In the United States, the term mixed-breed is a favored synonym over mongrel among people who wish to avoid negative connotations associated with the latter term. The implication that such dogs must be a mix of defined breeds may stem from an inverted understanding of the origins of dog breeds. Purebred dogs have been, for the most part, artificially created from random-bred populations by human selective breeding with the purpose of enhancing desired physical, behavioral, or temperamental characteristics. Dogs that are not purebred are not necessarily a mix of such defined breeds. Therefore, among some experts and fans of such dogs, mongrel is still the preferred term.

Dog crossbreeds, sometimes called designer dogs, also are not members of a single recognized breed. Unlike mixed-breeds, crossbreed dogs are often the product of artificial selection—intentionally created by humans—whereas the term mongrel specifically refers to dogs that develop by natural selection, without the planned intervention of humans.

The words cur, tyke, mutt, and mongrel are used, sometimes in a derogatory manner. There are also regional terms for mixed-breed dogs. In the United Kingdom, mongrel is the unique technical word for a mixed-breed dog. North Americans generally prefer the term mix or mixed-breed. Mutt is also commonly used in the United States and Canada. Some American registries and dog clubs that accept mixed-breed dogs use the term All-American to describe mixed-breed dogs.

There are also names for mixed-breeds based on geography, behavior, or food. In Hawaii, mixes are referred to as poi dogs, although they are not related to the extinct Hawaiian Poi Dog. In the Bahamas and the Turks and Caicos Islands, the common term is potcake dogs (referring to the table scraps they are fed). In South Africa, the tongue-in-cheek expression pavement special is sometimes used as a description for a mixed-breed dog. In Trinidad and Tobago, these mixed dogs are referred to as pot hounds (pothong). In Serbia, a similar expression is prekoplotski avlijaner (over-the-fence yard-dweller). In Russia, a colloquial term дворняга (yard-dweller) is used most commonly. In the Philippines, mixed-breed street dogs are often called askal, a Tagalog-derived contraction of asong kalye (”street dog"), while in Singapore, they are known as Singapore Specials. In Puerto Rico, they are known as satos; in Venezuela they are called yusos or cacris, the latter being a contraction of the words callejero criollo (literally, street creole, as street dogs are usually mongrels); and in Chile and Bolivia, they are called quiltros. In Costa Rica, it is common to hear the word zaguate, a term originating from a Nahuatl term, zahuatl, that refers to the disease called scabies. In the rural southern United States, a small hunting dog is known as a feist. In Yucatan Peninsula, Mexico, they are called "malix" (ma.liʃ), meaning "no breed" in Yucatec Mayan.