Objective Previous studies of military veterans have produced mixed findings regarding whether combat exposure is directly related to suicidal ideation or is indirectly related to suicidal ideation via its influence on other factors. The present study used a longitudinal design to test the hypothesis that post-traumatic stress disorder (PTSD) symptom severity mediates the effect of combat exposure on suicidal ideation in veterans. Method Participants included 319 post-9/11 veterans (83.4% male; 42.1% White/52.1% Black; Mage = 39.7) assessed at baseline, 6, and 12 months. Structural equation modeling and bootstrapped confidence intervals were employed to examine the direct and indirect relationships between combat exposure, suicidal ideation, and PTSD symptom severity. Results Results from the mediation model, in which demographic variables and non-combat trauma were included as covariates, revealed that the indirect effect of combat exposure on suicidal ideation via PTSD symptom severity was statistically significant, accounting for 64.1% of the covariance between combat exposure and suicidal ideation. Conclusions This study provides longitudinal evidence that the effects of combat exposure on suicidal ideation are mediated by PTSD symptom severity, suggesting the importance of targeting such symptoms in treatment to mitigate suicide risk among veterans with combat exposure.