Male–male interactions in mixed-sex groups of social mammals are typically characterized by a mix of hostility and affiliation, as a result of inherent conflicts over mating opportunities, and the costs and benefits of social alliances, co-operative behaviors, and coalitionary defense. In species of nonhuman primates that form all-male groups, it is still unclear how the tradeoffs between the benefits of forming an all-male group and the cost of male–male competition in seeking mating opportunities with females in bisexual groups influence social cohesion in different seasons. Here, we used social network analysis to quantify the impact of reproductive seasonality on social cohesion and clique size of bachelor males residing in an all-male unit (AMU) in wild black-and-white snub-nosed monkeys (Rhinopithecus bieti). These primates are strict seasonal breeders and live in a modular social system composed of a number of one-male units (OMUs) and an associated peripheral AMU. We found that the AMU social network had a significantly lower density, centralization, clustering coefficient, and smaller clique size during the mating season compared to the non-mating period. However, aggression among AMU males during both mating and non-mating periods was low. Our results suggest that network structure topology in male same-sex social units is modulated by seasonal changes. Bachelor males engage in two types of competition to gain reproductive success: first, which is analogous to contest competition, in which bachelor males act aggressively and challenge OMU leader males in an attempt to take over an OMU; and second, which is more analogous to scramble competition, in which bachelor males avoid aggressive interactions and instead engage in sneaky copulations with fertile females. Our work adds to an understanding of the maintenance of all-male groups in species that form a multilevel society.