Collaborative, or participatory governance is an increasingly common means of addressing natural resource issues, especially in the American West where patchworks of public, private, and tribal interests characterize the region's resources. In this context, unlikely alliances, or partnerships among diverse actors who have historically been at odds, have a growing potential to shape social and ecological outcomes, for better or worse. While these unlikely alliances have received greater attention in recent years, relatively little research has worked to synthesize the concept across diverse contexts and disciplines. Based on a review of the literature on unlikely alliances in natural resource governance, we develop a framework that synthesizes the individual motivations and contextual factors that influence their formation, as well as the social and ecological outcomes that they create. We use this framework to analyze six illustrative cases of unlikely alliances. Our analysis of these cases suggests that unlikely alliances in the American West are likely to arise in the presence of a crisis, when appropriate leadership is present, when some of the actors have interacted effectively in the past, and when actors need to pool resources. The cases also illustrate some common outcomes, including environmental improvement, transformation of social networks, policy change, and shifts in power relationships. We discuss the implications of unlikely alliances for the social-ecological future of the American West. Our paper highlights the role of unlikely alliances in shaping patterns of natural resource governance, and provides a focus for further research in this realm.