The indigo bunting is a smallish songbird, around the size of a small sparrow. It measures long, with a wingspan of . Body mass averages , with a reported range of . During the breeding season, the adult male appears mostly a vibrant cerulean blue. Only the head is indigo. The wings and tail are black with cerulean blue edges. In fall and winter plumage, the male has brown edges to the blue body and head feathers, which overlap to make the bird appear mostly brown. The adult female is brown on the upperparts and lighter brown on the underparts. It has indistinct wing bars and is faintly streaked with darker markings underneath. The immature bird resembles the female in coloring, although a male may have hints of blue on the tail and shoulders and have darker streaks on the underside. The beak is short and conical. In the adult female, the beak is light brown tinged with blue, and in the adult male the upper half is brownish-black while the lower is light blue. The feet and legs are black or gray.