Recent evidence suggests that several extinct primates, including contemporaneous Paranthropus boisei and Theropithecus oswaldi in East Africa, fed largely on grasses and sedges (i.e., graminoids). As the only living primate graminivores, gelada monkeys (Theropithecus gelada) can yield insights into the dietary strategies pursued by extinct grass- and sedge-eating primates. Past studies of gelada diet were of short duration and occurred in heavily disturbed ecosystems. We conducted a long-term study of gelada feeding ecology in an intact Afroalpine ecosystem at Guassa, Ethiopia. Geladas at Guassa consumed ≥56 plant species, ≥20 invertebrate species, one reptile species, and the eggs of one bird species over a 7-year period. The annual diet consisted of 56.8% graminoid parts, 37.8% forb parts, 2.8% invertebrates, and 2.6% other items, although geladas exhibited wide variability in diet across months at Guassa. Edible forbs were relatively scarce at Guassa but were strongly selected for by geladas. Tall graminoid leaf and tall graminoid seed head consumption correlated positively, and underground food item consumption correlated negatively, with rainfall over time. Geladas at Guassa consumed a species-rich diet dominated by graminoids, but unlike geladas in more disturbed habitats also ate a diversity of forbs and invertebrates along with occasional vertebrate prey. Although graminoids are staple foods for geladas, underground food items are important “fallback foods.” We discuss the implications of our study, the first intensive study of the feeding ecology of the only extant primate graminivore, for understanding the dietary evolution of the theropith and hominin putative graminivores, Theropithecus oswaldi and Paranthropus boisei. Am J Phys Anthropol 155:1–16, 2014. © 2014 Wiley Periodicals, Inc.