Written patient education materials are essential to motivate and help patients to participate in their own care, but the production and management of a large collection of high-quality and easily accessible patient education documents can be challenging. Ontologies can aid in these tasks, but the existing resources are not directly applicable to patient education. An ontology that models patient education documents and their readers was constructed. The Delphi method was used to identify a compact but sufficient set of entities with which the topics of documents may be described. The preferred terms of the entities were also considered to ensure their understandability. In the ontology, readers may be characterized by gender, age group, language, and role (patient or professional), whereas documents may be characterized by audience, topic(s), and content, as well as the time and place of use. The Delphi method yielded 265 unique document topics that are organized into seven hierarchies. Advantages and disadvantages of the ontology design, as well as possibilities for improvements, were identified. The patient education material ontology can enhance many applications, but further development is needed to reach its full potential.