Indicator 6.5.2 of the Sustainable Development Goals is defined as the ‘proportion of transboundary basin area with an operational arrangement for water cooperation’. While the establishment of such an indicator is most opportune and promising, its formulation and calculation raise technical, scientific and legal questions. This article primarily focuses on these legal issues. This manuscript assesses the indicator, addresses some of its key shortcomings, and offers a concrete and detailed proposal for an adjustment of its methodology and reporting mechanism, ahead of the 2020 revision of the SDG indicators by the Inter-agency and Expert Group on SDG Indicators.