The purpose of this study was to assess the content, construct, and discriminative validity and responsiveness of the Patient-Specific Functional Scale (PSFS) in patients with thumb carpometacarpal arthritis. Data were collected at Xpert Clinics, comprising 34 outpatient hand surgery and hand therapy clinics in the Netherlands. We included 267 patients for content validity and 323 patients for construct validity and responsiveness. The PSFS items were classified into the International Classification of Function Core Set for Hand Conditions to assess content validity. We used hypothesis testing to investigate the construct validity and responsiveness. The Michigan Hand Outcomes Questionnaire was used as a comparator instrument. The standardized response mean was calculated to evaluate the magnitude of change. For discriminative validity, we used independent t tests to discriminate between satisfied and dissatisfied patients. We classified 98% of the PSFS items in the International Classification of Function "activities" and "participation" domains, indicating good content validity. Two of six hypotheses for construct validity and three of six hypotheses for responsiveness were confirmed. The standardized response mean for the PSFS was 0.57 (0.46-0.68) and 0.47 (0.35-0.58) for the Michigan Hand Outcomes Questionnaire total score. The mean PSFS score showed good discriminative validity because it could distinguish between satisfied and dissatisfied patients at the 3-month follow-up. The PSFS scores showed good content and discriminative validity in patients with first carpometacarpal arthritis. Hypothesis testing for responsiveness and construct validity indicates that the PSFS measures a unique construct different from the Michigan Hand Outcomes Questionnaire. The PSFS may be a useful scale for measuring the patient-specific status of individuals with thumb carpometacarpal arthritis.