Male and female Red-legged Seriemas look alike, although males are slightly larger than females. Both weigh about 1.5 kg, and stand approximately 90 cm. Their most striking feature is the crest, which is unique among South American birds. The crest is comprised of permanently raised slightly stiff feathers at the base of the bill which can reach 10 cm in length. Seriemas are one of the few birds with eyelashes.
Seriemas have loose plumage, especially on the neck and underparts.
They are mainly gray in coloration, with shades of brown, except the abdomen which is white. The wing and tail feathers are banded black and white. The skin surrounding the eye is blue and the iris of the eye is yellow. The bill and legs are both red. Being mostly ground dwelling birds, seriemas have three short, very sharp front toes, and a raised, slightly smaller hind toe.