Abstract: Social challenges may have driven the evolution of intelligence in primates and other taxa. In primates, the social intelligence hypothesis is supported by evidence that primates know a lot about their own relationships to others and also know something about the nature of relationships among other individuals (third-party relationships). Knowledge of third-party relationships is likely to play an especially important role in coalitions, which occur when one individual intervenes in an ongoing dispute involving other group members, by helping individuals to predict who will support or intervene against them when they are fighting with particular opponents, and to assess which potential allies are likely to be effective in coalitions against their opponents. To date, however, there is no evidence that primates make use of knowledge of third-party relationships when they form coalitions. Here, I show that male bonnet macaques, Macaca radiata , use information about third-party rank relationships when they recruit support from other males. Males consistently chose allies that outranked themselves and their opponents, and made such choices considerably more often than would be expected by chance alone. The analysis shows that the data do not fit simpler explanations based upon males' knowledge of their own relationships to other males or males' ability to recognize powerful allies.