Measuring the quality of information delivered to patients with severe mental illness (SMI), i.e., schizophrenia, bipolar disorders, and major depressive disorders, is essential to improve their quality of care. In this work, we described the different steps of the validation of the PREMIUM computerized adaptive testing (CAT) for measuring the quality of information delivered to patients with SMI. The PREMIUM item bank regarding information included 25 items. A total of 499 patients with schizophrenia (53%), bipolar disorders (26%), and major depressive disorders (22%) were recruited from numerous in- and outpatient settings in France. Unidimensionality, local independence, and monotonicity were verified for 19 items of the item bank. The psychometric properties were satisfactory for both internal (RMSEA = 0.069, CFI = 0.969, TLI = 0.963) and external validity (in particular, significant associations were found with age, educational level, and social functioning). The CAT exhibited satisfactory accuracy and precision (standard error of measurement <0.55 and root mean square error <0.3), with an average administration of eight items. This CAT may be used by healthcare professionals in psychiatric settings to accurately assess the patients’ experience with information.