What explains the design and development of funding rules at international organizations? I investigate this question in the context of the United Nations system, which has undergone a dramatic shift in financing. Long associated with mandatory contributions, the United Nations increasingly relies on voluntary resources earmarked by individual donors. Previous studies have investigated the financing puzzle from a behavioral perspective and have found that wealthy donors use voluntary funding to rein in costs and constrain international organization programs. Providing an alternative theoretical approach, I investigate the financing puzzle from an institutional design perspective. I provide original United Nations funding rule data to demonstrate that it is not only funding practices, but also underlying funding rules, that have changed over time. I theorize how states with favorable views of the United Nations that sought to expand its activities — rather than those that desired to constrain it — had incentives to introduce funding rules that offered more flexibility and control to donors. I test the argument with a longitudinal case study of funding rule design and change at United Nations economic development institutions. The article expands the institutional design literature by integrating funding rules as a consequential design component and provides a novel explanation for changes in United Nations financing.