Ateline monkeys, the largest primates in the Neotropics, may disperse more than one million seeds/km2/d at sites where they are abundant, but it is unclear whether a reduction in their populations can alter plant diversity patterns. The species richness and composition of regenerating plants as a proxy of future plant communities were studied by comparing 16 sites with different ateline abundance in three countries in northwestern South America. A total of 3658 plots included 94,340 regenerating plants, which were assigned to species or morphospecies. Paired t-tests comparing sites in the same region but with different densities of atelines, and regression analyses showed a consistent positive relationship between ateline density and plant diversity. These results were due to the larger number of stems per area and higher evenness at sites with more atelines, suggesting higher recruitment rates for dispersed seeds. Differences were also found in plant composition, as canopy, endozoochorous, and medium seed size plants were consistently more abundant in sites with more ateline monkeys than in sites with less atelines. The findings of this study suggest that these primates play a key role in plant regeneration. In order to maintain the diversity and plant composition of tropical forests for future generations, conservation of these large frugivores and other key game species is imperative.