Cognitive dysfunction is an important feature of schizophrenia and bipolar disorder. There is uncertainty about the relative magnitude of cognitive deficits in these disorders. We evaluated a total of 446 individuals: 229 with schizophrenia, 117 with bipolar disorder, and 100 controls without a history of psychiatric disorder. All participants were administered the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS), a cognitive screening battery that evaluated immediate verbal memory, visuospatial/constructional abilities, attention, language, and delayed memory. A comparison of the three groups showed significant differences on the RBANS total score and all of the measured domains. In all of the comparisons, the schizophrenia group obtained the lowest scores, followed by the bipolar disorder group, and then the individuals without psychiatric disorder. In an analysis of covariance of RBANS total scores with the patient samples, the difference between schizophrenia and bipolar disorder remained significant after controlling for a range of demographic and clinical variables. Both schizophrenia and bipolar disorder are associated with significant cognitive impairments, but those in schizophrenia are more severe. Cognitive deficits may be an appropriate target of treatment interventions in these disorders.