The aim of this study was to develop and investigate psychometric properties of the Health Promotion Intervention Questionnaire, a newly developed instrument intended to measure patients' subjective experiences of health promotion interventions in the mental health services. Health promotion practice has often been contested; therefore, it is important to produce convincing evidence that health promotion initiated by the health-care services actually works. A cross-sectional study was performed in order to investigate health promotion interventions among mental health service outpatients. A sample of 135 patients was used to test the psychometric properties of a new questionnaire designed to measure subjective experiences of health promotion interventions carried out by the mental health services. The factor structure of the questionnaire was tested by means of principal component factor analysis with varimax rotation. Reliability was measured in terms of internal consistency of the factors and test–retest reliability in a subsample of patients. The final scale contained 19 items derived from four factors: alliance, empowerment, educational support, and practical support. In terms of internal consistency, reliability was found to be good, both for the overall scale and the subscales. Test–retest reliability was moderate or higher in 13 out of 19 items and poor in only one item. This questionnaire, developed to evaluate patients' experiences of health promotion interventions in mental health services, may be used as an alternative in further empirical studies. With additional testing, the questionnaire could facilitate the work of defining and evaluating health promotion interventions conducted by the mental health services.