The first Masterplan for the Gambia River Basin Development Organisation (OMVG) was recently co-constructed through extensive consultation, participatory workshops, and hydro-economic modelling. Regional stakeholders and experts elaborated sectoral plans reflecting national priorities and defined five development scenarios. Participatory modelling led through iteration to agreeing to a sixth trade-off scenario capable of irrigating 64,000 ha, producing 4180 GWh/year of hydropower while satisfying low flow requirements to maintain ecosystem services. Combined with continuous training on water modelling, the development of this masterplan enhanced regional knowledge of water resources and supports OMVG in its ability to make informed water allocation and management decisions.