Using museum specimens from Mexico, Canada, and the western United States, we examined sexual dimorphism in the Wilson's Warbler (Wilsonia pusilla), a Nearctic-Neotropic migrant (Passeriformes: Parulidae). On average, males had longer tails, wing chords, and eighth and ninth primaries than females. Three methods for quantifying cap plumage showed that differences in cap size and pattern alone could not definitively separate the sexes. Discriminant functions are presented for sexing individuals using cap category, cap length, wing chord, tail length, and ninth primary length. More specific functions are provided for samples from Alaska and eastern Mexico. For each group, equations are included for assigning individual probabilities of belonging to either sex.