BACKGROUND: In individuals with severe mental illness (SMI), low muscle strength heightens the risk of mortality and chronic disease development. Routine muscle strength assessments could identify vulnerabilities, thereby reducing the growing burden associated with SMI. However, integration into clinical settings faces obstacles because of limited resources and inadequate healthcare staff training. The 5 sit-to-stand (5-STS) test offers an alternative for measuring muscle strength compared with more complex or demanding tests. Nevertheless, its validity in individuals with SMI remains unexplored. AIMS: This study aimed to analyse the criterion validity of the 5-STS test in SMI, considering potential age, gender and body mass index influences. METHOD: In a cross-sectional study following the 'STrengthening the Reporting of OBservational studies in Epidemiology' (STROBE) guidelines, 82 adults with SMI (aged 18-65, 24 women) were assessed. Participants underwent both the 5-STS test and the isometric knee extension strength (KES) test. RESULTS: Analysis revealed a significant moderate correlation coefficient and intraclass correlation coefficient (-0.58 for both) for all participants, indicating that the measures are valid and assess related aspects of the same construct. Strong agreement was observed in women and the older age groups. The 5-STS test demonstrated accuracy, with a standard error of estimate lower than the within-subject variability on the KES test. Bland-Altman plots showed limits of agreement values of -3.39 and 3.52 for the entire sample, and heteroscedasticity analyses indicated consistent differences between the 5-STS and KES tests across all groups analysed, except in the women's group. CONCLUSIONS: The 5-STS test seems to be a valid test for assessing muscle strength in individuals with SMI, supporting its usefulness for routine assessment in clinical settings, facilitating detection and intervention in critical situations.