ABSTRACT: The ability to identify the sex of individuals is essential in studies of ecology, behavior, and conservation, but reliable methods for sexing species that exhibit low sexual dimorphism are often time consuming or invasive. Previous studies have evaluated the usefulness of morphological measurements as easy and minimally invasive means of sexing seabirds in the field. We used a discriminant function analysis (DFA) to determine the accuracy of sexing Galápagos penguins Spheniscus mendiculus using 6 morphological measurements: bill depth, bill length, head length, gape, flipper length, and foot length. Using these variables, we sexed 95% of study penguins correctly. Simplified functions, including bill depth and length, or bill depth only, also correctly classified the sex of 95% of study penguins. We also looked for sexual dimorphism in plumage, estimating the size of the white feather patch underneath the chin. Ninety-five percent of penguins with little to no white chin patch were female, while penguins with larger chin spots were both male and female. We show that Galápagos penguins, a rare and Endangered seabird, may be sexed accurately, even when data are limited to 1 morphological measurement.