As cities face escalating interlinked challenges, including air pollution, heat stress, and unequal access to greenspace, there is an urgent need for data-driven, community-led solutions. Urban ReLeaf empowers communities and public authorities to tackle urban greening, heat stress, and air pollution through citizen-driven environmental monitoring. Six pilot cities are pioneering a new model for climate resilience and inclusive policymaking. Utrecht (NL) enables citizens to track urban heat and to shape better strategies for cooler, healthier neighbourhoods. Cascais (PT) invites citizens to assess greenspaces to keep their city cool. Dundee (UK) is co-developing greener, more accessible spaces with residents. Mannheim (GER) harnesses citizen data to address its heat action plan. In Athens (GR), a participatory tree registry is closing data gaps and reshaping greenspace management. Riga mobilizes communities to monitor air pollution and greenspace use, ensuring policies reflect citizens’ needs. By embedding citizen science into policy, Urban ReLeaf connects communities, data, and governance, helping to transform cities into greener, climate-resilient spaces for all.