Corporate water stewardship, i.e., proactive water-using corporate engagement in water management and governance, has been hailed as a solution to global water challenges. However, it has also aroused criticism and skepticism, as it has been feared to lead to private securitization of resources and institutional capture especially in locations with weak public institutions and regulation. This article tackles this “prisoner’s dilemma” of corporate water stewardship by exploring when and how it is legitimate considering the private nature of corporations and their power to change water management and governance processes and their outcomes. An analytical framework is constructed based on a literature review and applied into a case-study of corporations active in water stewardship initiatives in South Africa. The case-study findings suggest that the stewardship agenda would benefit from (1) a more open acknowledgement of power asymmetries between corporations and other parties; (2) more careful and systematic evaluation and enhancement of legitimacy of corporations to engage in public good and common pool water resources in the first place; and (3) stewardship actions should support stronger public institutions and especially civil society to equally participate. The research community is called in to scrutinize and facilitate the multi-actor water governance processes, which include corporations to assist in the effort.