A growing body of literature suggests that the ratio of the lengths of the second to fourth digits (2D:4D) on human hands is sexually dimorphic and associated with prenatal exposure to gonadal hormones, circulating serum testosterone, and a number of psychological and behavioral measures. Little research has investigated digit ratios in nonhuman species. In the present study, we investigated sex differences in digit ratios and their possible association with serum testosterone in a captive group of Guinea baboons (Papio papio). Contrary to the sex difference typically reported in humans, male baboons exhibited a substantially larger 2D:4D than did female baboons. Consistent with the human data, however, lower 2D:4D was associated with higher serum testosterone among the males. The present findings suggest that the relationship between digit ratios and male gonadal hormones may be phylogenetically well-conserved, although they also suggest possible species differences in the causal relationships between developmental mechanisms and sex-differentiated digit length patterns.