This paper explores how uncertainty shapes transboundary water governance. It considers how a politics of uncertainty emerges alongside other political strategies, such as the politics of scale and knowledge, where uncertainty is not only the result of incomplete data and analysis, but can also reflect diverse normative stances, as a part of transboundary hydropolitics. The paper takes as a case study the Xayaburi Dam, a case with a high degree of uncertainty. The paper concludes that the politics of uncertainty holds implications for trans-border environmental justice. Presented at the International Conference on Development and Cooperation of the Mekong Region, 4-5 December 2014, Seoul.