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European cities face escalating pressures from air pollution, heat stress, biodiversity loss, and unequal access to greenspaces, alongside widening social inequalities. Urban ReLeaf, a Horizon Europe project, positions citizen science as a means of generating inclusive, fine-grained environmental data to support climate-resilient urban planning. Through pilot activities in Athens, Cascais, Dundee, Mannheim, Riga, and Utrecht, the project explores how different models of citizen engagement and data collection can enrich environmental research, address local data gaps, and inform evidence-based decision-making. Each city co-designs participatory pilot campaigns aligned with its environmental challenges and policy priorities. Across several pilots, residents contribute high-frequency data using wearable sensors to capture detailed patterns of urban heat exposure, complementing official monitoring systems. Beyond heat-related data, city-specific campaigns focus on a range of environmental themes. In Dundee, families, students, and community groups assess greenspace quality, accessibility, and use, generating insights that inform inclusive park upgrades and long-term greenspace strategies. In Riga, residents collect air quality data to support targeted greening and mobility-related interventions in traffic-intensive neighbourhoods. Athens and Mannheim focus on participatory tree registries, where citizens and municipal staff jointly document street trees, their condition, ecosystem services, and social value. These registries feed into municipal asset management systems, strengthening tree stewardship, transparency, and urban forestry planning. In Cascais, residents document environmental comfort and public use of parks and greenspaces to inform urban design and adaptation measures, while in Utrecht citizen thermal comfort perceptions and measurements are integrated into municipal planning tools to support cross-departmental decision-making. Across these diverse contexts, Urban ReLeaf demonstrates how citizen science can generate high-density environmental datasets that add value to official data while strengthening collaboration between communities, researchers, and public authorities. Iterative co-design processes foster trust, shared ownership of data, and pathways for sustained institutional use. At the same time, the pilots show that differences in data applicability, uptake, and institutional integration can vary across domains and urban contexts. In this presentation, we introduce the Urban ReLeaf project as a cross-city case study showing how citizen science can connect environmental research with urban planning and decision-making. Drawing on pilot activities in six European cities, we present co-designed approaches that combine participatory methods and digital tools. We highlight selected city campaigns focused on greenspace perceptions, air quality monitoring, and participatory tree registries driving integration of citizen observations into municipal planning tools, illustrating how locally tailored citizen science activities can complement official data and inform concrete urban actions.