We examine the social underpinnings of intergroup effectiveness—the effectiveness with which pairs of teams perform on joint tasks. In adopting a relational social network perspective, we propose that strong ties within and across team boundaries assume center-stage in facilitating the collaborative performance of resource-interdependent teams. In support of our hypotheses, longitudinal, multi-source data from 370 employees working in 93 teams of a Spanish public service organization suggest that strong ties within and across team boundaries interactively predict managerial intergroup effectiveness ratings of resource-interdependent teams. We discuss theoretical implications with respect to effective intergroup relations, the performance benefits of strong ties, and the design of team-based work.