Abstract: Purpose: While social support is widely viewed as a protective factor against posttraumatic stress disorder (PTSD), few studies have directly tested whether social support buffers the long-term effects of pre-existing PTSD symptoms or baseline combat exposure among Veterans (i.e., the stress-buffering hypothesis). Methods: To address this gap, the current study tested perceived social support as a moderator of the effects of baseline PTSD symptoms and combat exposure on PTSD symptoms at 10-year follow up in a sample of post-911 Veterans (N = 783). Results: Higher levels of combat exposure and baseline PTSD symptoms predicted elevated PTSD symptoms at 10-year follow-up. Perceived social support moderated these effects, such that the impacts of baseline symptoms and combat exposure were attenuated for Veterans with high levels of perceived support. However, buffering effects were less evident at higher levels of combat exposure and were not significant at very high levels of baseline PTSD symptoms. Conclusion: While findings are broadly consistent with the stress-buffering hypothesis, results of the present study suggest that the benefits of perceived social support may be less evident at higher levels of combat exposure. Results also offer preliminary evidence that perceived social support is less protective for Veterans with severe pre-existing symptoms.