Primate diets are fundamental in understanding their behavioral responses to habitat, and ecological plasticity and adaptation across and within species. Although species belonging to the genus Macaca are frugivores, inter- and intra-specific differences in diets are considerable. Macaques inhabiting low-latitude tropical forests rely more heavily on fruits than those in temperate forests. We studied two groups of Assamese macaques (Macaca assamensis) in a limestone forest of the Nonggang Nature Reserve, Guangxi, China from July-September 2012. We collected data using instantaneous scan sampling methods to investigate dietary composition and temporal variation across daytime hours. We observed that macaques consumed a total of 45 plant species, which included 30 tree, 3 shrub, 11 vine, and 1 herb species. On an average, macaques consumed 22.3 plant species per month. Young leaves and fruits were staple foods, accounting for 52.4% and 46.1% (21.3% for immature fruits and 24.8% for mature fruits) of the total diet, respectively, whereas the remainder (including 0.9% flowers, 0.3% mature leaves, and 0.3% other items) accounted for a negligible proportion. Young leaves of Bonia saxatilis, a shrubby, limestone-endemic bamboo, represented 43.8% of the diet. Macaques did not strictly select foods according to species biomass in the forest. The bulk diet (85.5%) comprised only nine plant species of > 2% each. Compared with the populations inhabiting other forests, Assamese macaques inhabiting the karst forest consumed more young leaves and less fruits. This variation was linked to lower fruit abundance in the karst forest, which could be caused by latitude and annual rainfall. Moreover, dietary composition varied during daytime hours. Macaques consumed more fruits in the early morning (7:00), and 100% young leaves during midday (13:00-14:00). Additionally, we found strong relationships between the dietary composition and ambient temperature. Consumption of young leaves increased with increasing ambient temperature, whereas consumption of both mature fruits and total fruits decreased with decreasing ambient temperature. In summary, the Assamese macaques in the karst forest could be regarded as folivorous, while still preferring fruits during the rainy season. Together with temporal variation in their diets, these macaques exploited an energy balance strategy, which enabled them to adapt more efficiently to their unusual karst habitat. To the best of our knowledge, this is the first report on the temporal variation in dietary composition of Assamese macaques, providing further information towards understanding their ecological plasticity.