Body mass is fundamental for understanding growth, health, and aspects of life history but records of body mass are rarely available for wild primates. We documented the body mass of all individuals in a group of wild bearded capuchin monkeys (Sapajus libidinosus) at annual intervals for seven consecutive years. Sexual dimorphism in body mass was more pronounced than reported in the literature for adults in this genus: females in our sample were relatively light (average 2.1 kg), while males had average body mass (3.5 kg). Three other notable differences between males and females were evident. First, males grew more rapidly and for a longer period than females. We estimate that males attained full body mass at 9.8 years of age and females at 7.5 years. Second, males showed greater inter‐individual variability than females in growth rates and adult mass. Third, males gained about 20% above their baseline body mass upon becoming alpha, and lost that amount when they lost that status, but body mass in females was unrelated to social status. We also report preliminary data on mass and age of natal males at dispersal and mass and age at first reproduction for one female. The pattern of sexual dimorphism in ontogeny and inter‐individual variability in body mass in bearded capuchins suggests different competitive risks in the two sexes commensurate with a mating system characterized by female choice of mates in multi‐male, multi‐female groups. Am. J. Primatol. 78:473–484, 2016. © 2015 Wiley Periodicals, Inc.