Identifying cancer patients who are experiencing psychosocial challenges during the early phase of oncology treatment can prevent escalating patient distress. Standardized screening methods allow the medical team to identify those at high risk for poor adjustment. The purpose of this study was to provide preliminary psychometric evidence for a brief, self-administered screening instrument, the Psycho-Oncology Screening Tool (POST), designed to evaluate emotional and physical distress, depressive symptoms, and social concerns. Participants included 944 radiation oncology patients who completed the POST prior to their treatment appointment and a subgroup of 516 patients who completed the POST and one of six concurrent validity measures. Psychometric analysis included construct validity evidence provided through confirmatory factor analysis (CFA), internal reliability estimates, and concurrent validity estimates assessed with bivariate correlations between POST subscales scores and conceptually similar established measures. A three-factor CFA model was found to produce acceptable model fit, supporting the three domain structure of the POST. Furthermore, the three subscales-emotional and physical distress, depressive symptoms and social concerns-were found to produce acceptable internal reliability estimates (α = .73-.88). Concurrent validity evidence was observed with significant, moderate to large correlations between the POST subscales and all relevant measures (i.e., Profile of Mood States, Hospital Anxiety and Depression Scale, Functional Assessment of Cancer Therapy Fatigue, Beck Depression Inventory, Pain Disability Index, and the Interpersonal Support Evaluation List) with correlations ranged from 0.42 to 0.83, p < 0.01. Patients and clinic staff reported no problems administering or completing the POST. Results of this study support the psychometric soundness as well as the feasibility and acceptability of the POST as a brief screening tool for oncology patients receiving outpatient services.