Cognitive theory posits that core beliefs play an active role in developing and maintaining symptoms of depression, anxiety, and psychosis. This study sought to comprehensively examine core beliefs, their dimensionality, and their relationships to depression, anxiety, and attenuated psychotic symptoms in two groups of community youth: a group at ultrahigh risk for psychosis (UHR; n = 73, M age = 18.7) and a matched healthy comparison group (HC; n = 73, M age = 18.1). UHR youth reported significantly more negative beliefs about self and others, and significantly less positive beliefs about self and others. HC youth rarely endorsed negative self-beliefs. Exploratory factor analyses found that HC negative self-beliefs did not cohere as a single factor. We hypothesized specific links between core beliefs and symptoms based on cognitive models of each disorder, and tested these links through regression analyses. The results in the HC group were consistent with the proposed models of depression and anxiety. The results in the UHR group were consistent with proposed models of depression and negative psychotic symptoms, somewhat consistent with a proposed model of positive psychotic symptoms, and not at all consistent with a proposed model of anxiety. These findings add to a growing developmental literature on core beliefs and psychopathology, with important clinical implications.