This paper describes a semantic model for urban sustainability and its implementation as an ontology. The ontology functions as an integration layer for the heterogeneous data sources used to calculate the set of sustainability indicators. These indicators are integrated in the semantic model so they can be indexed by geographical area, semantic concept, or key sustainability category, and they can be accessed through the ontology vocabulary. Users can explore and query both the model and the instance data mapped to it. At a more abstract level, this approach can help with understanding the potentially contradicting effects of changes in the values of the parameters that are modeled, running alternative scenarios and comparing options, and ultimately, selecting solutions that optimize the performance metrics of interest.