The GREENGAGE project aims to promote innovative governance processes, help public authorities in shaping their climate mitigation and adaptation and associated policies in the context of European Green Deal. To achieve this aim, the GREENGAGE project leverages citizens’ participation and equips citizens with innovative digital solutions that aim to transform citizen’s engagement and cities’ effectiveness in delivering the European Green Deal objectives for carbon neutral cities. Focusing on mobility, air quality and healthy living, the aim is to inspire citizens to observe and co-create their cities by sensing their urban environments. The objective is to complement, validate, and enrich information in authoritative data held by the public administrations and public agencies. This will be facilitated by engaging with citizens to co-create green initiatives and to develop Citizen Observatories. In GREENGAGE, Citizen Observatories (COs) will facilitate innovation pilots in co-examining environmental issues, integrating novel bottom-up process with top-down perspectives. This will provide the basis to co-create and co-design innovative solutions to monitor environmental problems with the help of citizens. With two interrelated project dimensions, the project aims to enhance intelligence applied to city decision-making processes and governance by engaging with citizen observations integrated with Copernicus, GEOSS, in-situ, and socio-economic intelligence, and by delivering innovative governance models based on novel decision-making methodologies and technologies. The proposed citizens observatory campaigns will be deployed and demonstrated in 5 pilot engagements in selected cities and regions including: Bristol (the United Kingdom), Ørestad, Copenhagen (Denmark), Turano (Italy), Gerace (Italy) and the region of NorthBrabant (the Netherlands). These innovation pilots aim to highlight the need for smart city governance by promoting citizen engagement, co-creation, gathering new data which will complement existing datasets supporting evidence-based decision and policymaking.