Humans often exhibit more certainty than is justified. Drawing on recent work in Form Theory, a theory of conceptual structure, the present paper proposes and tests a novel hypothesis about one important source of psychological certainty. On Form Theory, concepts have a central form: a core structure within each concept that stores a subset of its information, gives that information default influence in cognition, and activates a distinctive set of strong form-generated beliefs. The present paper tests the proposal that psychological certainty is one such form-generated belief. In Experiment 1, participants judged which properties would be present in newly encountered instances of familiar concepts. While they generalized both form and non-form properties to new instances, they reported being certain only for form properties. In Experiment 2, participants showed the same pattern for newly learned concepts. As soon as a property had been encoded as part of a concept’s central form, participants were much more likely to report being certain of its presence in a new instance than when the same property had been introduced in a generic sentence, but not as part of the concept’s central form. These findings help reveal a deep connection between psychological certainty and the structure of human concepts. Further, they suggest that conceptual structure is one important source of psychological certainty and provide a framework for investigating whether strongly held beliefs documented in research on stereotyping, memory, clinical depression, and conflict may sometimes derive from how concepts have been structured.