The article explores the theme of collaborative construction of territorial knowledge through the use of ICTS, proposing a new approach to spatial representation based on semantic ontologies. The theoretical perspective is applied in a recent experimentation conducted in collaboration between the Politecnico and the Università di Torino, called OnToMap – Community Maps 3.0. The essay is divided into two parts. The first part aims to define the theoretical framework of the eParticipation practices, specifying which approach to participation is assumed as conceptual background. The second part focuses on the web-based application OnToMap, which relies on an territorial ontology for encourage construction of collective and shared knowledge of the places. It is therefore proposed a definition of “Social Semantic Mapping” based on the integration of GIS tools, VGI practices and Web 3.0 applications. The article ends with a reflection on semantic and cartographic representation of urban space and its potential in terms of citizen empowerment.