The purpose of this study was to compare the psychometric properties of the Screen for Cognitive Impairment in Psychiatry (SCIP) when applied to patients diagnosed with schizophrenia (n = 126) or bipolar I disorder (n = 76), and also to compare the cognitive impairment in both samples of patients and a control group (n = 83) using the SCIP and a complete neuropsychological battery. The SCIP is a scale intended to quickly and easily assess cognitive impairment in patients with severe psychiatric disorders. The results showed firstly that, in terms of internal consistency, temporal stability, dimensional structure, and criterion-referenced validity, the SCIP provides reliable and valid scores at an equivalent level in both schizophrenia and bipolar I disorder samples. Secondly, it showed that differential cognitive impairment between the two patient groups occurs only in verbal memory, although the effect size of the difference is small. Finally, compared with the control group, cognitive impairment was present at all levels in both groups of patients using both the SCIP and the neuropsychological battery, which indicates that the SCIP is a good screening tool for cognitive deficits in schizophrenia and bipolar and useful in clinical practice for healthcare professionals.