We present the PESCaDO Ontology, a modular application ontology exploited for personalized environmental decision support, that enables to formally describe (i) the user decision support request, (ii) the environmental data relevant to process the request, as well as (iii) the decisions and conclusions to be produced. The PESCaDO Ontology was thoroughly developed following state of the art best practices, and it is matched with a comprehensive and detailed documentation.