Phosphorus (P) is a critical natural resource for food production, but one that is subject to global supply vulnerabilities. P is also responsible for endemic eutrophication in waterbodies due to poor stewardship in the food chain. Catchments are natural social-ecologically bounded systems for P use in agriculture and water management. Stakeholders, such as farmers, water and sewerage service companies, local authorities, and environmental organisations mediate catchment adaptive capacity to P supply risks and P pollution in waterbodies. Adaptive capacity at this level has been insufficiently explored in addressing the P challenge, yet is essential to it. We address this gap by exploring through a qualitative study of stakeholders in two United Kingdom catchments. Our results suggest that the awareness and relevance of P-supply challenges is low in catchments, but the problem of waterbody vulnerability to excess P is of greater concern. Our findings highlight the roles in adaptive capacity of entrenched practices; knowledge and training activities and organisations; stakeholder cooperation and synergy; funding, infrastructure, and technology; the governance environment; and time needed to draw down P. We find that farmers and water companies are especially important to adaptive capacity as they directly interact with P flows. We therefore suggest that catchment adaptive capacity would be significantly improved through a well-supported, and expanded package of existing efforts such as providing scientific evidence of catchment P dynamics; training; payments; more empowered local governance. This effort would support catchment stakeholders to adopt effective P-stewardship practices within a multi-decade integrated catchment management strategy.