Posttraumatic stress disorder (PTSD) patients have a high comorbidity with type 2 diabetes (T2D). Whether PTSD influences the risk of diabetes is still not known. We used GWAS data from European ancestry of PTSD (23,121 cases and 151,447 controls) and T2D (80,154 cases and 853,816 controls) to investigate the bidirectional associations between PTSD and T2D by the Mendelian randomization (MR) analysis. We showed that PTSD was causally associated with higher odds of T2D (OR = 1.04, 95% CI: 1.01-1.06, P = 0.0086), but not vice versa. Our study suggests that PTSD may increase the risk of T2D. PTSD sufferers should be screened for T2D and its precursor known as metabolic syndrome.