Transboundary hydropower dams are sources of hydroelectricity supply that sit in shared international rivers or watersheds, and/or generate benefits and costs that flow across national borders. Scholars have been exploring the impacts of hydropower dams at a local, regional and national scale for decades, however the transboundary impacts of hydropower ventures have been less studied. Nonetheless, the advent of a new hydropower boom, where a large proportion of untapped hydropower potential lies in transboundary settings, means that there is a need to better understand the specific benefits and costs in those contexts to foster more equitable and just outcomes, and to better examine the dynamics shaping the future of hydroelectricity. To depict the state-of-the-art within this critical field of research, we conduct a systematic review of 1264 peer-reviewed articles published on transboundary hydropower dams from 2009 to 2019. We find that most studies in our sample focus on issues related to water management and water allocation, whereas fewer focus on the scope of hydropower benefits, their temporal and spatial variation, and equity and justice dimensions. Moreover, there is minimal exploration of how differences in relative economic and financial capabilities can impact the distribution of transboundary hydropower benefits. Whether transboundary hydropower dams lead to optimal outcomes is highly dependent on underlying benefit sharing arrangements as well as an explicit acknowledgement and tackling of governance asymmetries. The study concludes that there is an urgent need to systematically assess these conditions to favour just outcomes for all stakeholders.