The second-to-fourth digit ratio (2D : 4D) is a well-studied sexual dimorphism that likely arises as a result of prenatal androgenic effects on homeobox gene expression. This dimorphism has been found to exhibit interesting phylogenetic patterns in which females have larger ratios than males among most mammalian species and males have larger ratios than females in most avian and reptilian species. Digit ratio has not been measured in more basal taxa such as amphibians. In this study, 2D : 4D and snout–vent length (SVL) were measured in 40 male and 44 female adult Oophaga [=Dendrobates] pumilio frogs to investigate the existence and patterning of sexual dimorphism. The 2D : 4D of the rear feet varied significantly by sex with males having a larger ratio than females. The digit ratio of the front feet did not differ between the sexes. SVL or its interaction with sex did not affect 2D : 4D on any of the feet. These results indicate that 2D : 4D in an anuran is both sexually dimorphic and follows a pattern between the sexes similar to that of diapsid species. A potential application of 2D : 4D to environmental monitoring is also discussed.