Objectives To demonstrate the ability of the Screen for Cognitive Impairment in Psychiatry (SCIP-S) to discriminate between cognitively-impaired individuals and those with adequate functioning in a sample of schizophrenic and bipolar patients, as well as in a control group. Methods The SCIP-S, together with a full neuropsychological battery, was administered to three groups: patients with schizophrenia, patients diagnosed with bipolar disorder I, and controls. The battery scores were used to perform a standardization with respect to the control group and this served to determine the comparison groups (cognitively impaired versus unimpaired) for each of the subtests of the SCIP-S. A full analysis of decision validity was conducted on the basis of receiver operating characteristic curves (sensitivity and specificity, +LR and −LR, PPV and NPV). Results All the subtests yielded adequate values for sensitivity and specificity with the proposed cut-off points, while the total score of the SCIP (<70) was associated with a sensitivity of 87.9 and specificity of 80.6. Conclusions The SCIP-S shows adequate decision validity as a screening tool for cognitive deficit in patients diagnosed with schizophrenia or bipolar disorder.