Background The risk of suicide among Veterans is of major concern, particularly among those who experienced a combat deployment and/or have a history of PTSD. Design and methods This was a retrospective cohort study of post-discharge suicide among Vietnam-era Veterans who are members of the Vietnam Era Twin (VET) Registry. The VET Registry is a national sample of male twins from all branches of the military, both of whom served on active duty between 1964 and 1975. Military service and demographic factors were available from the military records. Service in-theater was based on military records; combat exposure and PTSD symptoms were assessed in 1987 by questionnaire. Mortality follow-up, from discharge to 2016, is identified from Department of Veterans Affairs, Social Security Administration, and National Death Index records; suicide as a cause of death is based on the International Classification of Death diagnostic codes from the death certificate. Statistical analysis used Cox proportional hazards regression to estimate the association of Vietnam-theater service, combat exposure, and PTSD symptoms with suicide while adjusting for military service and demographic confounding factors. Results From the 14,401 twins in the VET Registry, there were 147 suicide deaths during follow-up. In adjusted analyses, twins who served in the Vietnam theater were at similar risk of post-discharge suicide compared with non-theater Veterans; there was no association between combat and suicide. An increase in severity of PTSD symptoms was significantly associated with an increased risk of suicide in adjusted analyses (hazard ratio = 1.13 per five-point increase in symptom score; 95% CI: 1.02–1.27). Conclusions Service in the Vietnam theater is not associated with greater risk of suicide; however, PTSD symptom severity poses a degree of risk of suicide in Vietnam-era Veterans. Adequate screening for PTSD in Veterans may be promising to identify Veterans who are at increased risk of suicide.