In social species, conflict with outsiders is predicted to affect within-group interactions, and thus influence group dynamics and the evolution and maintenance of sociality. While empirical evidence exists for a relationship between out-group conflict and intragroup behaviour in humans, experimental tests in other animals are rare. In a model fish system, we show that simulated out-group intrusions cause post-conflict increases in intragroup affiliation, but no changes in post-conflict intragroup aggression. Post-conflict affiliation was greater following intrusions by neighbouring compared to non-neighbouring individuals; neighbours represent greater threats to the dominance rank and breeding success of residents, and are visible in the aftermath of the intrusion. By providing strong evidence of a link between out-group conflict and post-conflict intragroup behaviour, and demonstrating that intragroup affiliation is affected by the nature of the out-group intrusion, our study shows the importance of considering post-conflict behaviour for our understanding of cooperation and social structure.