Urban Digital Twins (UDTs) promise to facilitate the transition to a smarter planning and decision-making process, but they face many challenges to meet stakeholders’ expectations and to support delivering a better living environment for citizens. Their full potential can only be reached through more flexible sharing and integration of data and data models. From this perspective, we argue that UDTs can be realised in an ecosystem of data spaces that support a federated data architecture. We present the conceptual architecture for UDTs in a federated data spaces ecosystem, describe the different components and layers, and reflect on the UDT mediator role in relation to other actors in the ecosystem. We aim to contribute to the field of UDTs by showing the way forward for their development, connected to the concepts of data spaces and federated database systems, and to the field of data spaces by introducing UDTs as a new component playing a mediator role.