Multistakeholder partnerships (MSPs) in global environmental governance are either praised for their problem-solving capacities and inclusion of various societal actors or criticized for their limited accountability and corporate dominance. Despite the lively scholarly debate and the continued promotion of MSPs by international organizations and governments, knowledge about how environmental MSPs are perceived by the public is very limited. Understanding the sources of public support for MSPs is important, given its crucial role in MSPs’ abilities to secure resources and achieve their goals. In this article, we evaluate whether and how institutional features of MSPs influence citizens’ legitimacy beliefs. Building on previous studies, we theorize which institutional dimensions of MSPs matter for citizens’ level of support. We conduct population-based survey experiments in Brazil, the United Kingdom, and the United States, encompassing more than 6,000 respondents. The results from the survey experiments have substantive implications for our understanding of the role of MSPs.