Mentalising, the capacity to understand behaviour via underlying mental states, is a key construct in psychopathology. While self-report instruments are widely used to assess mentalising, significant questions about their psychometric properties persist and no systematic review has comprehensively evaluated them using standardised criteria. This systematic review, guided by the COnsensus-based Standards for the selection of health Measurement Instruments () methodology, aims to: (1) identify all available self-report mentalising measures for adults; (2) evaluate the methodological quality of their validation studies; (3) synthesise and grade the quality of evidence on their measurement properties and (4) provide evidence-based recommendations for their use in research and clinical practice.

Five electronic databases (SCOPUS, Web of Science, PsycINFO, PubMed, ProQuest) will be searched from their inception, supplemented by a search of grey literature and reference lists. We will include studies of any design that report on at least one measurement property of a self-report measure of mentalising in adults. Two reviewers will independently screen all records, extract relevant data and assess the methodological quality of included studies using the COSMIN Risk of Bias checklist. For each instrument, the evidence for each measurement property will be synthesised, and the overall quality of the evidence will be graded using a modified Grading of Recommendations Assessment, Development and Evaluation approach.

As this systematic review will synthesise data from previously published studies, it does not require formal ethical approval. The findings will be disseminated through a peer-reviewed, open-access publication and presentations at scientific conferences. The results will provide a comprehensive inventory of available measures and a rigorous evaluation of their psychometric quality, creating an evidence base to guide clinicians and researchers in selecting the most appropriate instruments for mentalising assessment.

CRD420251031469.