Abstract. Due to their distinctive features, multiteam systems (MTSs) face significant coordination challenges—both within component teams and across the larger system. Despite the benefits of informal mechanisms of coordination for knowledge-based work, there is considerable ambiguity regarding their effects in MTSs. To resolve this ambiguity, we build and test theory about how interpersonal interactions among MTS members serve as an informal coordination mechanism that facilitates team and system functioning. Integrating MTS research with insights from the team boundary spanning literature, we argue that the degree to which MTS members balance their interactions with members of their own component team (i.e., intrateam interactions) and with the members of other teams in the system (i.e., inter-team interactions) shapes team- and system-level performance. The findings of a multimethod study of 44 MTSs composed of 295 teams and 930 people show that as inter-team interactions exceed intrateam interactions, team conflict rises and detracts from component team performance. At the system level, balance between intra- and inter-team interactions enhances system success. Our findings advance understanding of MTSs by highlighting how informal coordination mechanisms enable MTSs to overcome their coordination challenges and address the unique performance tension between component teams and the larger system.