The increasing complexity of urban planning projects today requires new approaches to better integrate stakeholders with different professional backgrounds throughout a city. Traditional tools used in urban planning are designed for experts and offer little opportunity for participation and collaborative design. This paper introduces the concept of geospatial tangible user interfaces (GTUI) and reports on the design and implementation as well as the usability of such a GTUI to support stakeholder participation in collaborative urban planning. The proposed system uses physical objects to interact with large digital maps and geospatial data projected onto a tabletop. It is implemented using a PostGIS database, a web map server providing OGC web services, the computer vision framework reacTIVision, a Java-based TUIO client, and GeoTools. We describe how a GTUI has be instantiated and evaluated within the scope of two case studies related to real world collaborative urban planning scenarios. Our results confirm the feasibility of our proposed GTUI solutions to (a) instantiate different urban planning scenarios, (b) support collaboration, and (c) ensure an acceptable usability.