Male and female Himalayan tahr Hemitragus jemlahicus exhibit extreme sexual size dimorphism and are spatially segregated outside the winter mating season. Segregation results in a greater change in both habitat selection and diet of males relative to females. Since the structure of the ruminant digestive tract is closely related to feeding habit, the rumen morphology of male and female tahr should reflect these apparently important seasonal changes. We therefore hypothesized that the rumen morphology of male tahr would undergo greater change between winter (when the sexes are aggregated) and summer (when the sexes are segregated) compared to females. Samples of rumen wall mucosa were cut from the dorsal rumen wall, atrium ruminis, caudo-ventral blindsac, and the ventral rumen wall of tahr collected during winter (eight male, 10 female) and summer (11 male, 10 female) from the Two Thumb Range, South Island, New Zealand. Although we recorded a significant seasonal increase in the mean surface enlargement factor (SEF) from winter to summer for both sexes, the SEF of females and males were not significantly different in winter (3.88 ± 0.55, SE, and 4.95 ± 0.61, respectively) or summer (7.71 ± 0.55 and 6.86 ± 0.52). We conclude that the effects of inter-sexual differences in habitat selection and diet on rumen morphology were insignificant relative to the extreme seasonal changes in forage quality and quantity that occur in the Two Thumb Range.