We performed a meta-analysis to assess the association of diabetes mellitus (DM) with the presence of thoracic aortic dissection (TAD) and/or thoracic aortic aneurysm (TAA). MEDLINE and EMBASE were searched through December 2015 using PubMed and OVID. For each study, data regarding DM prevalence in both the TAD/TAA and control groups were used to generate unadjusted odds ratios (ORs) for DM and 95% confidence intervals (CIs). Alternatively, an unadjusted and/or adjusted OR was directly abstracted from each individual study. Eleven eligible studies enrolling a total of 47 827 participants were included. A primary pooled analysis of all the 11 studies demonstrated that TAD/TAA was associated with significantly lower DM prevalence than controls (OR, 0.43; 95% CI, 0.31-0.59; P < .00001). Separate pooled analyses of 6 TAD case studies and 4 TAA case studies demonstrated TAD (OR, 0.34; 95% CI, 0.19-0.61; P = .0003) and TAA (OR, 0.65; 95% CI, 0.45-0.94; P = .02) were associated with significantly lower DM prevalence than controls. In conclusion, DM may be negatively associated with the presence of TAD/TAA.