The aim of this study was to develop and evaluate the Psychiatric Out-Patient Experiences Questionnaire (POPEQ). The instrument was developed following a literature review, patient interviews and pre-testing of questionnaire items. The POPEQ was administered as part of a postal survey of 15,422 adult outpatients attending Norwegian clinics; 6677 (43.3%) patients responded to the questionnaire. Items had low levels of missing data. Factor analysis showed that 11 widely applicable items contribute to a measure of overall experiences. Sub-dimensions include clinician interaction (six items) information (two items) and outcomes (three items). Item-total correlations ranged from 0.5 to 0.8. Cronbach's alpha and test-retest reliability estimates exceeded the criterion of 0.7; the majority were over 0.8 and total scores over 0.9. Construct validity was supported by the results of 128 tests. The POPEQ includes important aspects of patient experience for psychiatric outpatients and has excellent evidence for reliability and construct validity. The instrument is recommended for the measurement of psychiatric outpatient experiences.