A wealth of indicators and indicator sets, structured in the form of standards (e.g., ISO, OECD, SDG Index, etc.), have been developed and proposed over the past decades to define and measure urban sustainability. Unfortunately, literature studies confirm that there is a lack of knowledge systematisation for such indicator sets in the sustainability assessment domain. In this chapter, we present and describe a formal representation of knowledge about sustainability indicators and their interrelationships, through the development of a domain ontology. To increase the potential for its adaptability and reuse, as well as to facilitate its continuous redevelopment, we make the developed ontology available online, through a dedicated web portal.