Abstract: Intergroup interactions in social animals can vary from hostile to affiliative and may be influenced by factors such as competitive ability, resource values and existing intergroup relationships. Despite the potential for intergroup interactions to affect individual fitness and group stability, few studies have comprehensively tested how social, demographic and ecological factors may simultaneously influence intergroup interactions. Using 13 years of continuous data on intergroup interactions (n = 464), group composition, range use and diet, we investigated the factors that influenced the initiation and escalation of intergroup interactions in a fully habituated subpopulation of mountain gorillas in Volcanoes National Park, Rwanda. The majority of interactions were non-physically agonistic (57%), while peaceful exchanges and physical aggression were less common (18% and 25% of interactions respectively). Solitary males and young dominant silverbacks were the most likely to initiate an interaction, presumably because these males have the highest incentive to attract mates. Aggressive interactions between a group and a solitary male involved a high number of participating group members, reflecting the incentive to avoid injury and infanticide associated with solitary male encounters. Aggression between social groups escalated when groups were similarly sized, perhaps because these groups have similar competitive abilities. Peaceful intergroup interactions most commonly involved opponents that contained familiar and related individuals, suggesting that the short dispersal distance of gorillas may facilitate kin-selected intergroup tolerance. Variation and plasticity in gorilla behaviour during intergroup interactions are therefore dependent on the opponent’s familiarity and threat level. Significance statement: Intergroup interactions can vary from aggressive physical disputes to tolerant intermingling among extra-group individuals. Competition for access to limiting resources can often influence the occurrence of aggressive intergroup competition, but may not be sufficient to describe peaceful mingling between groups in social species. We found that although the main driver of intergroup interactions may be mating competition between males, interactions between familiar social groups were significantly more peaceful than interactions involving unfamiliar groups. These findings suggest that maintaining social and kin relationships with neighbouring groups may act as a strategy for reducing conflict in group-structured social species.