Philanthropic foundations play increasingly prominent roles in the environmental arena, yet remain largely under the radar of environmental governance scholars. We build on the small body of existing research on foundations in environmental governance to outline a research agenda on foundations as agents of environmental governance. The agenda identifies current understandings, debates, and research gaps related to three themes: 1) the roles foundations perform in environmental governance, 2) the outcomes of environmental philanthropy, and 3) the sources of foundation legitimacy. We call for more systematic and empirical research using diverse theoretical perspectives and methodological approaches. This research agenda will contribute to literature on agency in environmental governance by providing a more comprehensive picture of who governs the environment and how. Coming at a time when foundations are facing growing public scrutiny, it can also inform contemporary debates and offer practical insights for effective and equitable environmental philanthropy.