This study examined the prevalence, correlates and psychiatric comorbidity of Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) posttraumatic stress disorder (PTSD) in a nationally representative sample of U.S. veterans using data from the National Epidemiologic Survey on Alcohol and Related Conditions-III (n=3119 veteran respondents). The overall prevalence of lifetime PTSD was 6.9%. Lifetime PTSD prevalence was higher among veterans who were female (13.2%), aged 18–29 years (15.3%), Native American (24.1%) or Black (11.0%), previously or never married (9.6% and 11.2, respectively), had incomes less than $70,000 (7.2%–10.1%) and had >2 traumatic events (5.2%–14.7%). After adjusting for sociodemographic characteristics, comorbidity between lifetime PTSD and other psychiatric disorders was highest for any personality disorder (adjusted odds ratio [AOR] =11.1, 95% confidence interval [CI], 5.7, 21.5), any mood disorder (AOR=9.7, 95% CI, 4.6, 20.4) and any anxiety disorder (AOR=9.6, 95% CI, 5.1, 17.7), followed by nicotine, drug, and alcohol use disorders (AOR= 3.4, 95% CI,1.8, 6.5; AOR= 3.1, 95% CI, 2.0, 5.9; 2.1, 95% CI, 1.5, 3.1, respectively). Associations remained with any mood, anxiety, and personality disorders after controlling for other psychiatric disorders (AOR= 3.7, 95% CI, 1.2, 10.9; AOR= 3.5, 95% CI, 1.6, 7.4; AOR=4.5, 95% CI, 2.3, 8.7, respectively). Veterans who sought treatment for PTSD had more comorbid conditions, although treatment was only associated with comorbid drug use disorder (AOR=2.4, 95% CI, 1.0, 5.7). In U.S. veterans, PTSD is highly comorbid with other psychiatric disorders. Although many veterans remain untreated, comorbidity may influence treatment seeking.