Ejaculate traits vary extensively among individuals and species, but little is known about their variation among populations of the same species. Here, we investigated patterns of intraspecific variation in male reproductive investment in the terrestrial-breeding frog Pseudophryne guentheri. Like most anurans, breeding activity in P. guentheri is cued by precipitation, and therefore the timing and duration of breeding seasons differ among geographically separated populations, potentially leading to differences in the level of sperm competition. We therefore anticipated local adaptation in sperm traits that reflect these phenological differences among populations. Our analysis of six natural populations (two near the northern range edge and four central populations) revealed significant divergence in testes and ejaculate traits that correspond with latitude; males from the northern and dry edge of the species range had significantly smaller testes containing fewer, smaller and less motile sperm compared to those from the more mesic central populations. These findings may reflect spatial variation in the strength of postcopulatory sexual selection among populations, likely driven by local patterns of precipitation.