Question\hspace0.6em Although mental pain is present in many mental disorders and is a predictor of suicide, it is rarely investigated in research or treated in care. A valid tool to measure it is a necessary first step towards better understanding, predicting and ultimately relieving this pain. Study selection and analysis\hspace0.6em Following the Preferred Reporting Items for Systematic Reviews and Meta-\-Analyses guidelines, we performed a systematic review to identify all published standardised measures of mental pain. We used qualitative content analysis to evaluate the similarity of each measure, quantified via Jaccard Index scores ranging from no similarity (0) to full similarity (1). Finally, using the Consensus-\-based Standards for the selection of health Measurement INstruments (COSMIN) methodology, we evaluated each measure's development (assessing 35 features), its content validity (31 features) and if the latter was rated at least adequate, its other psychometric properties. Findings\hspace0.6em We identified 10 self-\-reported scales of mental pain in 2658 screened studies relying on diverse definitions of this construct. The highest average similarity coefficient for any given measure was 0.24, indicative of weak similarity (individual pairwise coefficients from 0 to 0.5). Little to no information was provided regarding the development and the content validity of all 10 scales. Therefore, their development and content validity were rated 'inadequate' or 'doubtful'. Conclusions and clinical implications\hspace0.6em There is not enough evidence of validity to recommend using one measure over others in research or clinical practice. Heterogeneous use of disparate measures across studies limits comparison and combination of their results in meta-\-analyses. Development by all stakeholders (especially patients) of a consensual patient-r\-eported measure for mental pain is needed. PROSPERO registration number\hspace0.6em CRD42021242679.