Patient-reported outcomes (PROs) refer to any report coming directly from patients about how they function or feel in relation to a health condition or its therapy. PROs have been applied in medicine for the assessment of the impact of clinical phenomena. Self-report scales and procedures for assessing physical pain in adults have been developed and used in clinical trials. However, insufficient attention has been dedicated to the assessment of mental pain. The aim of this paper is to outline the implications that assessment of mental pain may entail in psychiatry and medicine, with particular reference to a clinimetric index. A simple 10-item self-rating questionnaire, the Mental Pain Questionnaire (MPQ), encompasses the specific clinical features of mental pain and shows good clinimetric properties (i.e., sensitivity, discriminant and incremental validity). The preliminary data suggest that the MPQ may qualify as a PRO measure to be included in clinical trials. Assessment of mental pain may have important clinical implications in intervention research, both in psychopharmacology and psychotherapy. The transdiagnostic features of mental pain are supported by its association with a number of psychiatric disorders, such as depression, anxiety, eating disorders, as well as borderline personality disorder. Further, addressing mental pain may be an important pathway to prevent and diminish the opioid epidemic. The data summarized here indicate that mental pain can be incorporated into current psychiatric assessment and included as a PRO measure in treatment outcome studies.