Determining the sex of birds quickly in the field can help in studies of behavior and distribution, and when selecting particular sexes for deploying tracking devices or collecting samples. However, discerning males from females is difficult in species that are plumage monomorphic and have overlapping sexual-size dimorphism, as in Buff-breasted Sandpipers Calidris subruficollis. We developed three discriminant functions to sex Buff-breasted Sandpipers based on measurements of live birds captured in Brazil whose sex was confirmed with molecular techniques. We validated these discriminant functions using morphometric measures from other independent samples of known-sex live birds from wintering (Brazil), migration (Texas), and breeding (Alaska) sites. Discriminant functions derived from birds captured in Brazil accurately sexed ≥88% of the validation sample from Brazil, Texas, and Alaska. Errors in classification occurred among males on the wintering (0–5%) and breeding (8–12%) grounds, and females during migration (0–11%). Discriminant functions worked well because of the substantial sexual size dimorphism present in the species, with male traits being in general 5.2–10.4% larger than female traits. The size of morphological traits did not vary by age (after controlling for sex) for birds sampled on the wintering grounds and during migration. Our results indicate that discriminant functions can be used to sex after-hatch year (AHY) Buff-breasted Sandpipers throughout their range, and for hatch year (HY) birds during their first southbound migration and winter. Being able to accurately sex both AHY and HY birds using only morphological measurements will improve studies of the ecology and population structure of this species and enhance the application of conservation measures.