Bearded sakis (Chiropotes spp.) are among the least known primates in the world, particularly with regard to their social organization. However, extremely large and variable group size and growing evidence for affiliative relationships among males, make them a fascinating subject for studies of male–male tolerance and the evolution of large groups. During a 13-month study of free ranging Guianan bearded sakis (Chiropotes sagulatus) in continuous forest in Suriname, we documented variation in group size seasonally and social behaviour, particularly among males. We found that group size was smaller and more variable in the long dry season than the short wet season, perhaps in response to changes in resource availability. We found clear evidence of affiliative male–male relationships, with males being more social than females (89.2% of events involved one male versus 15.6% that involved one female) and typically socializing with other males (68.7% of events involved 2 or more males). We also found no evidence of hierarchical relationships between males (complete absence of agonistic interactions), and we documented unique male-affiliative behaviours and temporary all-male subgroups for the first time. To investigate potential explanations of male tolerance, we compared bearded saki social patterns to other multi-male group living primates (Ateline and Pan spp.). We found similarities with Brachyteles that suggest high intergroup mating competition and scramble competition for mates within groups.