The research aimed to assess the reliability, factor structure, and validity of the Italian adaptation of the Mentalization Scale (MentS), a 28-item self-report questionnaire that measures mentalization across three dimensions. The psychometric properties of the Italian version were examined in two studies with large samples of adults and adolescents. The first study (Study 1) aimed to evaluate, through exploratory and confirmatory factor analyses, the construct validity of the Italian version of the MentS in adolescents (N = 618) and adults (N = 720). The second study (Study 2) was undertaken to test the convergent validity and temporal stability of the Italian version of the MentS. Specifically, the study assessed the relationship between the MentS and scores on the Reflective Functioning Questionnaire (RFQ-8), one of the most widely used instruments to assess mentalization, in a large sample of high-school students (N = 472). Furthermore, the study evaluated the 4-week test-retest reliability of the instrument in a sample of undergraduates (N = 128). The questionnaire exhibited strong internal consistency across both adult and adolescent samples, with Cronbach’s alphas ranging from 0.71 to 0.83. Exploratory and confirmatory factor analyses consistently identified three correlated underlying factors within both age groups, demonstrating the robust factor structure of the Italian version of the MentS. Furthermore, the tool demonstrated strong convergent validity with the RFQ-8 and acceptable test-retest reliability over a 4-week period. These findings provide compelling evidence supporting the Italian version of the MentS as a reliable and valid self-report measure for comprehensively assessing different facets of mentalization.