The great-tailed grackle is a medium-sized, highly social passerine bird. 

Great-tailed grackles are medium-sized birds (larger than starlings and smaller than crows; 38 cm (15 in)-46 cm (18 in) with males weighing 203 g (7.2 oz)-265 g (9.3 oz) and females between 115 g (4.1 oz)-142 g (5.0 oz), and both sexes have long tails.

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.

Great-tailed grackles are noted for their diverse foraging habits.