This study aimed to define the concepts and learning objectives related to the polygons sub-learning area of the 7th - grade mathematics course as ontology-based and to develop a measurement tool based on this ontology within the framework of competency ontology. In this study, we determined and included the learning objectives in the ontology alongside the concepts and prerequisites. The most crucial difference that distinguishes this study from others is that it is objectively oriented. Throughout the study, we followed ontology development processes that met the system's requirements based on the ontology development. In this context, we saw that the ontology developed answered the intended competency questions. We think this study will make very important contributions in terms of bringing a different perspective to measurement and assessment in mathematics education.