The Roseate Spoonbill is 71–86 cm (28–34 in) long, with a 120–133 cm (47–52 in) wingspan and a body mass of 1.2–1.8 kg (2.6–4.0 lb). The tarsus measures 9.7–12.4 cm (3.8–4.9 in), the culmen measures 14.5–18 cm (5.7–7.1 in) and the wing measures 32.3–37.5 cm (12.7–14.8 in) and thus the legs, bill, neck and spatulate bill all appear elongated. Adults have a bare greenish head ("golden buff" when breeding) and a white neck, back, and breast (with a tuft of pink feathers in the center when breeding), and are otherwise a deep pink. The bill is grey. There is no significant sexual dimorphism.
Like the American Flamingo, their pink color is diet-derived, consisting of the carotenoid pigment canthaxanthin. Another carotenoid, astaxanthin, can also be found deposited in flight and body feathers. The colors can range from pale pink to bright magenta, depending on age and location. Unlike herons, spoonbills fly with their necks outstretched. They alternate groups of stiff, shallow wingbeats with glides.