The gray catbird (Dumetella carolinensis), also spelled grey catbird, is a medium-sized North American and Central American perching bird of the mimid family. It is the only member of the "catbird" genus Dumetella. Like the black catbird (Melanoptila glabrirostris), it is among the basal lineages of the Mimidae, probably a closer relative of the Caribbean thrasher and trembler assemblage than of the mockingbirds and Toxostoma thrashers. In some areas it is known as the slate-colored mockingbird.

The name Dumetella is based upon the Latin term dūmus ("thorny thicket"); it thus means approximately "small thornbush-dweller" or "small bird of the thornbushes". It refers to the species' habit of singing when hidden in undergrowth. The specific name carolinensis is Neo-Latin for "from the Carolinas".

The species was first described by Carl Linnaeus in his 1766 edition of Systema naturae. His original name Muscicapa carolinensis reflected the belief, widespread at that time, that the gray catbird was some sort of Old World flycatcher (presumably due to its remarkably plain coloration, not similar to other mimids).

The genus name has a convoluted nomenclatorial history. The monotypic genus Galeoscoptes, proposed by Jean Cabanis in 1850, was widely used up to 1907. This name roughly means "capped mockingbird", from Latin galea "helmet" and Ancient Greek skóptein (σκώπτειν, "to scold" or "to mock"). But as it turned out, Dumetella was a technically acceptable senior synonym, even though the peculiar circumstances of its publishing left the identity of its author unsolved until 1989. As it turned out, the genus name was published by C.T. Wood in 1837. His description is somewhat eccentric, and was published under his pseudonym "S.D.W.". Wood misquotes his source—John Latham's 1783 General Synopsis of Birds—as calling the bird "cat thrush", probably because he knew the species under that name from George Shaw's General Zoology. Latham's name was "cat flycatcher", analogous to the scientific name of Linné.

Shaw (and subsequently C.T. Wood) used Louis Pierre Vieillot's specific name felivox. This means "cat voice", a contraction of Latin felis ("cat") and vox ("voice"). Vieillot, differing from the earlier authors, believed the bird to be a true thrush (Turdus).

Though mimids were widely considered Turdidae until the 1850s, this was not any more correct than treating them as Old World flycatchers, as these three families are distinct lineages of the superfamily Muscicapoidea. In the mid-20th century, the Turdidae and even most of the Sylvioidea were lumped in the Muscicapidae—but the Mimidae were not.

Lastly, the smaller gray catbirds from Bermuda, which have proportionally narrow and shorter rectrices and primary remiges, were described as subspecies bermudianus ("from Bermuda") by Outram Bangs in 1901. But this taxon was never widely accepted, and today the gray catbird is generally considered monotypic as a species, too.

Adults weigh from 23.2 to 56.5 g (0.8 to 2.0 oz), with an average of 35–40 g (1.2–1.4 oz) They range in length from 20.5 to 24 cm (8.1 to 9.4 in) and span 22 to 30 cm (8.7 to 11.8 in) across the wings. Among standard measurements, the wing chord is 8.4 to 9.8 cm (3.3 to 3.9 in), the tail is 7.2 to 10.3 cm (2.8 to 4.1 in), the culmen is 1.5 to 1.8 cm (0.6 to 0.7 in) and the tarsus is 2.7 to 2.9 cm (1.1 to 1.1 in). Gray catbirds are plain lead gray almost all over. The top of the head is darker. The undertail coverts are rust-colored, and the remiges and rectrices are black, some with white borders. The slim bill, the eyes, and the legs and feet are also blackish. Males and females cannot be distinguished by their looks; different behaviours in the breeding season is usually the only clue to the observer. Juveniles are even plainer in coloration, with buffy undertail coverts.