This article conceptualizes role separation in multiple team membership (MTM) (i.e., the extent to which a multiteamer’s role within a focal team is different from his/her role in another team) as a key predictor of individual and team outcomes. Existing literature on MTM focuses primarily on the total number of concurrent teams that an individual contributes to, and thus largely ignores the potential diversity that may characterize MTM. In Study 1, we develop and validate a measure of MTM role separation that reflects differences in (a) expected work results; (b) team collaboration; (c) leader expectations; and (d) client and/or customer characteristics. In Study 2, we use field data to examine the cross-level implications of MTM role separation. As expected, we find that MTM role separation is positively related to a multiteamer’s role ambiguity within a focal team and, by extension, harms the performance of the entire focal team. This indirect relationship is not observed when a focal team’s teamwork quality (TWQ) is high. We discuss how these findings advance our understanding of the multifaceted and multi-level nature of MTM and help multiteamers, team leaders, and organizations deal with MTM’s challenges.