In the 'new Gilded Age' of mega-wealth and big philanthropy, academics are not paying enough attention to private foundations. Mirroring upward trends in philanthropy broadly, marine conservation philanthropy has more than doubled in recent years, reaching virtually every globally salient marine conservation issue in all corners of the planet. This paper argues that marine conservation philanthropy warrants a dedicated research agenda because private foundations are prominent, unique, and under-studied actors seeking to shape the future of a "frontier" space. We present a co-produced social science research agenda on marine conservation philanthropy that reflects the priorities of 106 marine conservation donors, practitioners, and stakeholders who participated in a research co-design process in 2018. These "research co-designers" raised 137 unique research questions, which we grouped into five thematic research priorities: outcomes, governance roles, exits, internal foundation governance, and funding landscape. We identify issues of legitimacy, justice, and applied best practice as cross-cutting research priorities that came up throughout the five themes. Participants from the NGO, foundation, and government sectors identified questions within all five themes and three cross-cutting issues, underscoring shared interest in this work from diverse groups. The research we call for herein can inform the practice of conservation philanthropy at a time when foundations are increasingly reckoning with their role as institutions of power in society. This paper is broadly relevant for social and natural scientists, practitioners, donors, and policy-makers interested in better understanding private philanthropy in any environmental context globally. • Private foundations are prominent, unique, and under-studied actors in marine conservation. • We co-produced a research agenda on ocean philanthropy with marine conservation donors, practitioners, and stakeholders. • Outcomes, governance roles, exits, internal governance, and funding landscape are key priorities. • There is also significant interest in issues of legitimacy, justice, and applied best practice. • Diverse groups share interest in identified research priorities. [ABSTRACT FROM AUTHOR]