Plastics in freshwater is of increasing concern to the public, given its potential negative impact on socio-ecological systems. At the same time, knowledge on monitoring plastics in freshwater is rather sparce at a global scale. Against this background, the United Nations Environment Programme (UNEP) has published the guidelines for the harmonization of methodologies for monitoring plastics in rivers and lakes. But while comprehensive guidelines exist, their implementation in different regional settings needs exemplification. The project ‘Monitoring plastics in freshwaters – A participatory approach: Phase II’ supports the example application of the plastics guidelines in three catchments in Asia (Hanoi, Vietnam), Africa (Accra, Ghana), and Europe (Rhine, Netherlands, Germany) through a socio-scientific companionship of the monitoring activities led by the Helmholtz Centre for Environmental Research (UFZ) and Wageningen University and Research (WUR). This socio-scientific companionship entails (i) an in-depth stakeholder analysis in the three case study areas, (ii) a pre-/post-test of monitoring capacities, (iii) workshop organization for knowledge sharing, and (iv) contributions to an overarching workflow document. As an expected outcome of the project, best practices for the implementation of the UNEP guidelines in most different geographical and social-political contexts are derived, to advance further from science to impact.