Sexual dimorphism in birds may express itself as differences in body size, plumage, color and/or behavior. Many species are monomorphic in color, which makes sex determination difficult in the field. In order to develop a tool to distinguish between male and female specimens of the White-faced Ibis, Plegadis chihi, by using external body measurements, the objective of this work was to quantify morphometric differences between sexes in adults of this species. The following variables were measured: culmen length, bill height and width, tarsus length, middle toe (with claw) length, wing chord, tail length and body mass. Males were larger than females in all of these variables, and presented statistically significant differences for six out of eight parameters. Three discriminant classification functions were obtained with an accurate total classification rate in >85% of the cases. The length of the culmen, tarsus, middle toe with claw, and wing chord, were among the most useful variables to discriminate between sexes. The classification functions are useful for the discrimination of sexes in the White-faced Ibis, with easy-to-take measurements. This information may be used by avian ecologists in future behavioral ecology, conservation biology, or evolutionary biology studies. The use of external morphometrics to sex monomorphic birds is of great value, being an inexpensive, less invasive and more immediate method.