Schema therapy (ST) has been proven to be an effective psychotherapy model in the treatment of complex personality disorders. ST helps analyze causes of emotional, cognitive, and behavioral alteration in patients using schemas and modes (i.e., sets of emotional states, coping responses and schemas active in a given moment). ST finds its empirical validation in the short Schema Mode Inventory (SMI), a practical tool consisting of 14 subscales assessing 14 different mode categories, grouped in 4 (child, coping, parent and adult modes) high-order categories used to assess different modes at different times. We introduced the Italian validation of the short SMI to a sample of 707 participants, of whom 230 were psychiatric patients. Confirmatory factor analysis (CFA) provides evidence that the 14-dimensional model best describes the SMI's structure at a lower level. Higher-order CFA provides evidence for both four higher-level mode categories and one higher-level mode category, the four-mode category being the best approach. Internal reliability, test-retest stability and the relationship between the SMI's subscales have been evaluated with promising results. Clinical vs nonclinical subjects were compared with a multigroup CFA in order to test invariance and with a MANOVA and Bonferroni post hoc comparisons in order to test mean differences. A linear thread was found for all modes except Bully/Attack. While contributing to international research and to the diffusion of SMI and schema therapy, our results also suggest that SMI is a powerful tool for the assessment of modes in ST, both in therapeutic and diagnostic contexts.