We used discriminant function analysis (DFA) of a series of external morphometric measurements, with confirmation by genetic methods, to develop a simple method to determine the sex of adult White-rumped Sandpipers (Calidris fuscicollis) wintering in southern Brazil. In total, we evaluated the sexual size dimorphism (SSD) of 82 males and 102 females captured with mist nets during the non-breeding period from 2010–2012 using eight morphological measurements. We found significant sex differences: females are significantly larger than males in bill and total head and wing lengths (% SSD 1.0–6.2%). The SSD in skull and bill lengths showed a tendency to maximize the bill length in females, while males exhibited a greater skull length than females (2.6%). Both males and females showed similar body mass, and heavier birds were found in the fall before the northward migration. Body condition showed no difference between the sexes but was higher in the fall than the spring. Molecular sexing showed that DFA would lead to misclassification of sex. Given this, the use of morphological measurements for sexing non-breeding birds of this species in the wintering area should be avoided.