Co-production processes enable urban planners and policymakers to unlock the potential of Nature-based Solutions (NbS) to address multiple urban challenges simultaneously. The Connecting Nature EU-funded project has worked with ten cities in Europe to foster the large-scale development of NbS while highlighting the benefits that NbS co-production can deliver. Based on the observation of these cities, this policy brief identifies six design principles for designing and evaluating co-production processes, and presents a set of recommendations to foster an enabling environment and support the mainstreaming of co-production into urban planning.