The great-tailed grackle or Mexican grackle (Quiscalus mexicanus) is a medium-sized, highly social passerine bird native to North and South America. A member of the family Icteridae, it is one of 10 extant species of grackle and is closely related to the boat-tailed grackle and the extinct slender-billed grackle. In the southern and southwestern United States, the grackle is sometimes referred to simply as a "blackbird" or (erroneously) a "crow" due to its glossy black plumage; however, grackles form their own unique genus that is separate from other "blackbirds", such as the red-winged and Brewer's blackbirds, despite being in the same family (Icteridae). Superficially, Brewer's blackbird is one of the most visually similar species to grackles.

In some parts of Mexico, the grackle is sometimes referred to in Spanish as cuervo ("raven"; "crow"), although it is not a member of the crow genus (Corvus) nor of their family (Corvidae).

The great-tailed grackle was formally described in 1788 by the German naturalist Johann Friedrich Gmelin in his revised and expanded edition of Carl Linnaeus's Systema Naturae. He placed it with the crows in the genus Corvus and coined the binomial name Corvus mexicanus. The type locality has been restricted to the region of Veracruz in Mexico. Gmelin's account was based on De Hocitzanatl that had been described by the Spanish naturalist Francisco Hernández de Toledo in his work Rerum medicarum Novae Hispaniae thesaurus, seu, Plantarum animalium mineralium Mexicanorum historia. The work was published in 1651, long after Hernández's death in 1587. "Hocitzanatl" was the name for the bird in a local Mexican language. The bird was also mentioned in 1770 by the French author Antoine-Joseph Pernety in his work Histoire d'un voyage aux Isles Malouines. The great-tailed grackle is now one of six species placed in the genus Quiscalus that was introduced by Louis Vieillot in 1816.

Eight subspecies are recognised:

Great-tailed grackles are medium-sized birds (larger than starlings and smaller than crows; 38–46 cm (15–18 in) with males weighing 203–265 g (7.2–9.3 oz) and females 115–142 g (4.1–5.0 oz), and both sexes have long tails. Wingspan ranging 48–58 cm (19–23 in). Males are iridescent black with a purple-blue sheen on the feathers of the head and upper body, while females are brown with darker wings and tail. Adults of both sexes have bright yellow eyes, while juveniles of both sexes have brown eyes and brown plumage like females (except for streaks on the breast). Great-tailed grackles, particularly the adult males, have a keel-shaped tail that they can fold vertically by aligning the two halves.

The great-tailed grackle and boat-tailed grackle were considered the same species until genetic analyses distinguished them as two separate species.

Great-tailed grackles have an unusually large repertoire of vocalizations that are used year-round. The sounds range from "sweet, tinkling notes" to a "rusty gate hinge". Males use a wider variety of vocalization types, while females engage mostly in "chatter", however there is a report of a female performing the "territorial song". Because of their loud vocalizations, great-tailed grackles are considered a pest species by some.

Great-tailed grackles originated from the tropical lowlands of Central and South America, but historical evidence from Bernardino de Sahagún shows that the Aztecs, during the time of the emperor Ahuitzotl, introduced the great-tailed grackle from their homeland in the Mexican Gulf Coast to the Aztec capital of Tenochtitlan in the highland Valley of Mexico, most likely to use their iridescent feathers for decoration. In more recent times, great-tailed grackles expanded their breeding range by over 5,500% by moving north into North America between 1880 and 2000, following urban and agricultural corridors. Their current range stretches from northwestern Venezuela and western Colombia and Ecuador in the south to Minnesota in the north, to Oregon, Idaho, and California in the west, to Florida in the east, with vagrants occurring as far north as southern Canada. Their habitat for foraging is on the ground in clear areas such as pastures, wetlands and mangroves, and chaparral. The grackles' range has expanded with agricultural and urban settings.