Background: Multimorbidity is a burden for the individual and to the healthcare sector worldwide, leading to a rising number of intervention studies towards this patient group. To measure a possible effect of such interventions, an adequate patient-reported outcome measure (PROM) is essential. The aim of this study was to assess the draft MultiMorbidity Questionnaire (MMQ), a PROM measuring needs-based quality of life and self-perceived inequity in patients with multimorbidity, for its psychometric properties and to adjust it accordingly to create a content- and construct valid measure. Methods: The draft MMQ was sent to 1198 eligible respondents with multimorbidity. Modern test theory and classical test theory were used to analyse data. Dimensionality of the suggested domains and invariance of the items were assessed through item analysis, examining the fit to a psychometric model. Results: The psychometric analyses were based on responses from 390 patients with multimorbidity. In the MMQ1, measuring needs-based QoL, evidence of six unidimensional scales was confirmed: physical ability (6 items), worries (6 items), limitations in everyday life (10 items), my social life (6 items), self-image (6 items), and personal finances (3 items). The psychometric analyses of the MMQ2 outlined four unidimensional scales measuring the feeling of Self-perceived inequity in patients with multimorbidity: experiences of being stigmatised (4–5 items), Experiences of insufficient understanding of the burden of disease (3 items), Experiences of not being seen and heard (4 items), Experience of powerlessness (5 items). These scales are relevant for patients’ with multimorbidity encounters with (1) their general pratitioner, (2) staff at their general practitioner’s surgery, (3) healthcare professionals, (4) staff at the local authorities and (5) friends, family, and others. Conclusion: The MMQ, a QoL measure for patients living with multimorbidity has been validated: the MMQ1 is a condition-specific PROM with adequate psychometric properties designed to measure needs-based QoL. The MMQ2 measuring Self-perceived inequity, has also been found to possess adequate measurement properties; however due to the risk of type 2 error a revalidation of MMQ2 is suggested.