Background: Posttraumatic stress disorder (PTSD) and problematic alcohol use commonly co-occur among military service members. It remains critical to understand why these patterns emerge, and under what conditions. Objectives: This study examined whether PTSD symptoms (PTSS) and alcohol involvement (quantity and frequency of use, heavy episodic drinking, and alcohol problems) are indirectly related through four distinct drinking motivations. A secondary aim was to identify factors, specifically forms of social support, which buffer these associations. Methods: Using baseline data from a randomized-controlled trial of health and well-being among civilian-employed separated service members and reservists, the present study examined these issues using a subsample of 398 current drinkers. Results: Parallel mediation models revealed PTSS–alcohol consumption associations were indirect through coping and enhancement motivations. PTSS was only related to alcohol problems through coping motivations. In addition, the indirect effect of PTSS on average level of consumption via coping motives was conditional on perceived support from friends and family, whereas the indirect effect for alcohol problems was conditional only on friend support. In contrast, the indirect effects of PTSS on alcohol consumption variables (but not problems) via enhancement motives were conditional on perceived support from friends and family. Conclusions/Importance: Future research and screening efforts should attend to individual motivations for drinking as important factors related to alcohol use and problems among service members experiencing PTSS, and emphasize the importance of communication, trust, and effective supports among military and nonmilitary friends and family.