The northern mockingbird (Mimus polyglottos) is a mockingbird commonly found in North America, of the family Mimidae. The species is also found in some parts of the Caribbean, as well as on the Hawaiian Islands. It is typically a permanent resident across much of its range, but northern mockingbirds may move farther south during inclement weather or prior to the onset of winter. The northern mockingbird has gray to brown upper feathers and a paler belly. Its tail and wings have white patches which are visible in flight.

The species is known for its ability to mimic bird calls and other types of sound, including artificial and electronic noises. Studies have shown its ability to identify individual humans and treat them differently based on learned threat assessments. It is an omnivore and consumes fruit, invertebrates, and small vertebrates. It is often found in open areas, open woodlands and forest edges, and is quite common in urbanized areas. The species breeds from southeastern Canada throughout the United States to the Greater Antilles. It is listed as a species of least concern by the International Union for Conservation of Nature.

The mockingbird is influential in United States culture, being the state bird of five states, appearing in book titles, songs and lullabies, and making other appearances in popular culture.

Swedish zoologist Carl Linnaeus first described this species in his Systema Naturae in 1758 as a species of thrush, Turdus polyglottos. Its current genus name, Mimus, is Latin for "mimic" and the specific polyglottos, is from Ancient Greek poluglottos, "harmonious", from polus, "many", and glossa, "tongue", representing its outstanding ability to mimic various sounds.

The closest living relative of the northern mockingbird is the tropical mockingbird (Mimus gilvus); the two form a superspecies, and occasional hybrids have been recorded where they overlap in Oaxaca and Veracruz in southeast Mexico.

There are three recognized subspecies for the northern mockingbird. There have been proposed races from the Bahamas and Haiti placed under the orpheus section.

The northern mockingbird is a medium-sized mimid that has long legs and tail. Males and females look alike. Its upper parts are colored gray, while its underparts have a white or whitish-gray color. It has parallel wing bars on the half of the wings connected near the white patch giving it a distinctive appearance in flight. The black central rectrices and typical white lateral rectrices are also noticeable in flight. The iris is usually a light green-yellow or a yellow, but there have been instances of an orange color. The bill is black with a brownish black appearance at the base. The juvenile appearance is marked by its streaks on its back, distinguished spots and streaks on its chest, and a gray or grayish-green iris.

Northern mockingbirds measure from 20.5 to 28 cm (8.1 to 11.0 in) including a tail almost as long as its body. The wingspan can range from 31–38 cm (12–15 in) and body mass is from 40–58 g (1.4–2.0 oz). Males tend to be slightly larger than females. Among standard measurements, the wing chord is 10 to 12 cm (3.9 to 4.7 in), the tail is 10 to 13.4 cm (3.9 to 5.3 in), the culmen is 1.6 to 1.9 cm (0.63 to 0.75 in) and the tarsus is 2.9 to 3.4 cm (1.1 to 1.3 in).