This paper deals with policy diffusion across countries in the context of collaborative river basin management. Borrowing from the literature on policy diffusion and ‘smart strategies’ of small European Union member states, we argue that policy diffusion is fostered by structural and agency‐related factors. We illustrate our theoretical ideas with a case study on the new Swiss policy on micropollutants in surface waters. Based on a rich set of documents and interviews with key actors, we show that the integration of Switzerland into formal and informal transnational networks fosters the diffusion of its micropollutants policy to other Rhine riparian countries. In addition, we analyse agency‐related factors favouring policy diffusion in river basins, such as a country's pioneer role, its expertise, the fact that its policy is in line with general policy goals in other countries and the political acceptance of the policy at home. Our analysis suggests that policy diffusion can be an important phenomenon in integrated water resources management. Copyright © 2016 John Wiley & Sons, Ltd and ERP Environment.