Purpose: Personality disorder (PD) assessment in older adults is challenging. In geriatric medicine, older adults with multi-morbidity are treated for their somatic, psychogeriatric, functional and social complaints and the presence of a PD can be a complicating factor in this treatment. Therefore, this study evaluates the diagnostic accuracy of a PD screening instrument, the Gerontological Personality disorder Scale (GPS) in a Dutch geriatric medicine population. Methods: Using an informant-based personality questionnaire (HAP) as a reference criterion, the psychometric properties of the GPS-informant version were assessed in a sample of N = 160 (62 male) outpatients (mean age = 81.7). Results: The internal consistency of the GPS (total score), Cronbach’s alpha, was α = 0.69. And the average inter-item correlation (total score) was 0.14. The test–retest reliability was r s = 0.68. The sensitivity and specificity for the GPS were 0.91 and 0.67, respectively. The GPS items showed predictive validity for PD status with 87.4% of predictions being accurate based on a logistic regression analysis. Conclusions: This is the first psychometric study to use the GPS as an age-specific screening instrument for PDs in Dutch geriatric medicine outpatients. The GPS is an adequate screening tool for PDs in geriatric medicine, given the high sensitivity. The diagnostic accuracy of the GPS-informant version is fair to excellent.