Objective This study aimed to validate the Korean version of a short screening tool for psychosis as the first stage in finding undiagnosed psychosis in the community. Methods The sample contained 126 consecutive psychiatric outpatients in National Medical Center, Seoul, Korea, between July 20 and July 22, 2020. The Psychosis Screener (PS) comprises 7 items covering psychotic symptoms. The presence of each psychotic symptom was determined by a trained mental health professional and coded “yes” or “no.” Two psychiatrists reviewed the medical records independently and extracted the ICD-10-based diagnoses. Any differences between the two clinicians were resolved by consensus, and the agreed diagnosis was used as a gold standard in the study. Results Among 126 psychiatric outpatients who were enrolled in a consecutive manner during the study period, the proportion of psychosis was 15.1%. The PS showed 78.9% sensitivity and 72.0% specificity when the optimal cut-off was 2, indicating that a score of 2 or more on the screener identified a likely case of psychosis. The area under the curve for the PS was 0.78 (95% CI: 0.67-0.87). Conclusion The Korean version of the PS has an ability to discriminate between those who meet the diagnostic criteria for psychosis and those who do not in a high-prevalence group.