The Laysan duck (American Ornithologists Union 1998), also known as the Laysan teal, is a small (38.1 to 43.2 centimeters [15 to 17 inches] in length, weight 420 to 500 grams [14.8 to 17.6 ounces]), mostly chocolate-brown duck with contrasting bi-colored body feathers (USFWS 1982; Moulton and Marshall 1996). It has an iridescent purplish-green speculum (wing patch) and a prominent white eye ring. There is considerable individual variation in plumage. The eye ring is nearly absent on juvenile birds and becomes more extensive and irregular in adults.

Leucism, or extensive white feathering, is common on the head and neck, especially in birds older than 3 years. The plumage of both sexes is quite similar, but bill and leg coloring can be used to distinguish sexes. In males, the short and spatulate bill is olive-green with black blotches along the maxilla (upper half of the bill). Females have a slightly shorter, paler orange bill with variable black mottling. Both sexes have dull orange legs, although the males legs typically are brighter (Moulton and Marshall 1996). Mass fluctuates significantly with season (Reynolds and Work 2005). Both males and females are lightest during the breeding season. Other morphometric characteristics (wing chord, tarsus, and bill length) are on average slightly larger for males (Moulton and Marshall 1996). On Laysan, molt typically occurs between July & August for males & between July & September for females (Moulton and Marshall 1996). For female ducks, the molt usually occurs after brood rearing. The timing of the molt is variable, as is the timing for breeding. It is a relatively long-lived species (12 years in the wild, 18 in captivity; Moulton and Weller 1984; Reynolds and Kozar 2000a) with a low reproductive rate in comparison with continental ducks. The Laysan duck is mostly nocturnal in its habits, feeds primarily on insects, and is very sedentary and terrestrial for a waterfowl species.