Nature Reviews Biodiversity, Published online: 30 January 2026; doi:10.1038/s44358-026-00136-2Defining non-native populations is essential yet challenging in freshwater systems, where policy relies on administrative borders and science on ecological units. This mismatch complicates regulation, risk assessment and biosecurity. We propose a basin-first, population-level classification that distinguishes intrabasin from interbasin nativity to better support coherent, ecologically grounded policy and management.