Previous reports have suggested a high prevalence of overweight and obesity among individuals with posttraumatic stress disorder (PTSD). Few studies, however, systematically analyze the relationship between PTSD and body mass index (BMI). We conducted a systematic review and meta-analysis aimed at estimating the association between PTSD and BMI. Fifty-four articles were reviewed, 30 of which (with 191,948 individuals with PTSD and 418,690 trauma-exposed individuals or healthy controls) were eligible for inclusion in the meta-analysis. The pooled standard mean difference, based on a random-effects model, was 0.41 (95% confidence interval, 0.28–0.54; z = 6.26; p < .001). Statistical heterogeneity between the included studies was high (p < .001; I2 = 99%). Despite limitations, the findings of this systematic review and meta-analysis suggest an association between PTSD and BMI. Furthermore, longitudinal studies tentatively indicate that PTSD may lead to an increase in BMI and, as such, to the development of overweight/obesity, particularly in women. Further prospective studies and research elaborating the nature and etiology of the association are required.