The lesser yellowlegs is a medium-large shorebird, 23–25 cm (9.1–9.8 in) in overall length and with a wingspan of 59–64 cm (23–25 in) and a weight of 67–94 g (2.4–3.3 oz). The sexes are similar both in plumage and in overall size. In breeding plumage, the upperparts are mottled with gray-brown, black and white. The underparts are white with irregular brown streaking on the breast and neck. In non-breeding plumage, the upperparts are more uniform gray-brown. The legs are yellow. Compared to the greater yellowlegs, the bill is shorter (visually about the same length as the head), slim, straight, and uniformly dark. The breast is streaked and the flanks are finely marked with short bars.

This species is similar in appearance to the larger greater yellowlegs, although it is more closely related to the much larger willet; the fine, clear, and dense pattern of the neck shown in breeding plumage indicates these species' actual relationships.

The call of this bird is softer than that of the greater yellowlegs.