Growing interest in Urban Green Spaces (UGSs) has led to the discovery of the wide spectrum of services that they either provide or support. Yet, there is persistent confusion about what types of green spaces exist and how they should be classified. Current Land Use (LU) and Land Cover (LC) maps both at European and national levels are either lacking or misclassify various types of UGS, thus underestimating the actual amount of green space that a city offers its citizens. In this paper, we highlight reasons for green space misclassification and suggest an ontology of UGSs. Our ontology takes into account both the physical appearance of UGSs and their semantics. We characterize the physical appearance using four different LC classes while providing semantics using unique characteristics expressed as rules. The ontology proposed emphasizes in particular frequently disregarded small and heterogeneous UGSs and can be used as a basis for their precise identification and mapping.