I examined how Sulawesi Tonkean macaques (Macaca tonkeana) respond in terms of their diet and activity patterns to anthropogenic habitat alteration in Lore Lindu National Park, Sulawesi, Indonesia. Anthropogenic habitat alteration comprised clearing of forest for agriculture and small-scale forest product collection. I quantified the diet and activity of 2 groups (CH and Anca), occupying habitats with different levels of alteration, via scan sampling. Tree abundance, key food specific density, and fruit production were greater in the minimally altered habitat (CH), substantiating the characterization of the group’s habitat as higher quality. For the group in the heavily altered habitat (Anca), alternative foods, e.g., insects, fungus, young and mature leaves, shoots, and stems, accounted for a significantly greater proportion of the diet. Dietary diversity is significantly lower in the Anca group, with 52% of their diet being palm fruits from Arenga pinnata. The activity patterns of the Anca group—more time foraging, less time moving, and more time resting than the CH group—reflect the lower resource availability in their habitat and their reliance on more alternative food items, coupled with their extremely small group size (6–9 individuals). The group may be at the optimal size in which foraging efficiency is maximized for the habitat, a response, in conjunction with dietary and behavioral flexibility, to alteration of their habitat. The results are contextualized with respect to the conservation value of human-modified landscapes.