Managing nature-based solutions (NBS) in urban areas for carbon mitigation and biodiversity outcomes is a global policy challenge, yet little is known about how to both assess and weave diverse knowledge systems and values into carbon-biodiversity trade-off assessments. This paper examines the spatial relationships between biophysical and social values for carbon sequestration potential (measured as carbon dioxide, CO2, flux) and biodiversity in Helsinki, Finland, using integrated valuation. The approach combines methods from carbon sequestration modelling, expert scoring approaches to biodiversity assessment and public participation geographic information systems (PPGIS). Results indicate strong spatial associations between biophysical assessment of CO2 flux and biodiversity priorities, and weaker associations between biophysical and social values. Integration of social and biophysical values leads to multiple pathways for protection of NBS to achieve carbon mitigation and biodiversity outcomes, as well as options for the spatial targeting of education and capacity building programs to areas of local concern.