This article reviews how global hydropower assemblages catalyze socioecological change in the world’s rivers. As a quintessential megaproject, massive dams and the hydropower they generate have long captivated the modernist development imaginary. Yet, despite growing recognition of the socio-ecological consequences of hydropower, it has recently assumed a central role in supporting renewable energy transitions. We highlight three trends in hydropower politics that characterize global hydropower assemblages: mega-dams as markers of nation-state development; river protection by territorial alliances and social movements opposed to hydropower; and transitions from spectacular, centralized hydropower installations to the propagation of small and large hydropower within climate mitigation schemes. We offer insights on how global hydropower assemblages force examination beyond traditional categories of “mega” through more holistic and grounded analyses of significance.