International funding for climate change action in developing countries may enhance the legitimacy of global climate governance. However, by allowing for a fragmented approach to mobilizing funds, current multilateral commitments raise further legitimacy challenges. We analyze the potential for unilateral and coordinated approaches to advance “output” and “input” legitimacy respectively by raising adequate funds and representing interests in contributing and recipient countries that are affected by funding decisions. Achieving legitimacy will require coordinated approaches to goal-setting, oversight and effort-sharing. Vesting contributing countries with substantial discretion over funding sources may enhance taxpayers’ support and boost funding more rapidly. However, multilateral coordination will be necessary to maximize opportunities for raising revenue from carbon pricing and to minimize adverse impacts of funding choices on developing countries. Our analysis provides a principled justification for the degree of fragmentation compatible with achieving legitimacy. These insights may inform future evaluation of legitimacy requirements in other spheres of environmental governance.