Background Few studies have longitudinally examined predictors of posttraumatic stress disorder (PTSD) in a nationally representative sample of US veterans. We examined predictors of warzone‐related PTSD over a 25‐year span using data from the National Vietnam Veterans Longitudinal Study (NVVLS). Methods The NVVLS is a follow‐up study of Vietnam theater veterans (N = 699) previously assessed in the National Vietnam Veterans Readjustment Study (NVVRS), a large national‐probability study conducted in the late 1980s. We examined the ability of 22 premilitary, warzone, and postmilitary variables to predict current warzone‐related PTSD symptom severity and PTSD symptom change in male theater veterans participating in the NVVLS. Data included a self‐report Health Questionnaire survey and a computer‐assisted telephone Health Interview Survey. Primary outcomes were self‐reported PTSD symptoms assessed by the PTSD Checklist for DSM‐5 (PCL 5) and Mississippi PTSD Scale (M‐PTSD). Results Predictors of current PTSD symptoms most robust in hierarchical multivariable models were African‐American race, lower education level, negative homecoming reception, lower current social support, and greater past‐year stress. PTSD symptoms remained largely stable over time, and symptom exacerbation was predicted by African‐American race, lower education level, younger age at entry into Vietnam, greater combat exposure, lower current social support, and greater past‐year stressors. Conclusions Findings confirm the robustness of a select set of risk factors for warzone‐related PTSD, establishing that these factors can predict PTSD symptom severity and symptom change up to 40 years postdeployment.