Referring to simultaneous competition and cooperation between firms, coopetition is emerging in practice as a promising source of value creation. However, the scholarly literature is dominated by a widespread assumption that opportunism, a core behavioral assumption of transaction cost economics, hinders value creation and requires formal safeguards in coopetition. The assumption of heightened opportunism in coopetition is at odds with rising adoption in practice, which often proceeds without formal safeguards. This raises concerns about the utility of existing theory for explaining coopetition dynamics and their implications for value creation. Building from theories of competitive dynamics and the resource-based view, my research challenges the dominant assumption of heightened opportunism and develops an alternative explanation to better explain coopetition dynamics. I identify and test informal market-based safeguards which reduce opportunism in coopetition. This provides theoretical resolution for conflicting findings in the literature and develops a nuanced understanding of the factors affecting opportunism in coopetition at multiple levels. It addresses the failure of extant research to explain coopetition dynamics and establishes foundations for systematic analysis of coopetition benefits and costs in future research. For managers, my findings move beyond simplistic perceptions that have emphasized instability, knowledge leakage, and the resultant need for formal safeguards in coopetition. Instead, I identify an efficient and effective alternative for constraining opportunism. This indicates that establishing, maintaining, and ultimately achieving value creation in coopetition relationships may be less challenging and costly than the literature assumes. Given the benefits of coopetition for both firms and society, this has important economy-level implications.