Objectives: This study aimed to systematically review and meta-analyze the reliability of the International Trauma Questionnaire (ITQ) and its child/adolescent version (ITQ-CA) in diverse populations, including clinical and non-clinical groups. Methods: A systematic search of the Scopus, Web of Science, and PubMed databases was conducted using predefined inclusion criteria.Studies reporting Cronbach’s α values for ITQ or ITQ-CA total scores or subscales (post-traumatic stress disorder [PTSD] and disturbances in self-organization [DSO], respectively) were included. Quality was assessed using the National Institutes of Health Quality Assessment Tool. A reliability generalization meta-analysis was conducted using a random-effects model to evaluate internal consistency.Heterogeneity was assessed using Cochran’s Q and I 2 statistics, and publication bias was examined using Egger’s test and funnel plots. Results: Sixty-two studies with 37766 participants from 28 countries were included. The ITQ-18 demonstrated high reliability, with a total score Cronbach’s α of 0.896 (k=33; 95% confidence interval [0.8773–0.9129]), PTSD subscale α=0.852 (k=45), and DSO subscale α=0.869 (k=45). The ITQ-CA showed strong internal consistency with a total score α of 0.887 (k=7) and subscale α values of 0.819 for PTSD (k=9) and 0.900 for DSO (k=9). Although the ITQ-22 was analyzed in fewer studies (k=4), it demonstrated promising reliability.High heterogeneity (I2 >80%) was observed, but no significant publication bias was detected. Conclusion The ITQ and ITQ-CA are reliable tools for assessing PTSD and complex PTSD in diverse populations. The findings underscore their psychometric robustness, but the marked heterogeneity highlights the need for contextual adaptations. Future research should expand on longitudinal studies and assess underrepresented regions to enhance cross-cultural validity.