Symptoms of posttraumatic stress (PTS) show significant comorbidity with alcohol use, but little is known about the mechanisms that might account for this comorbidity. Deficits in reward functioning have long been implicated in alcohol misuse and more recently in PTS reactions, but no study has examined whether reward deprivation may serve as a transdiagnostic risk factor for comorbid PTS-alcohol misuse. The current cross-sectional study sought to test the behavioral economic hypothesis that reward deprivation would be related to both PTS symptoms and alcohol problems, and would mediate the relation between PTS symptoms and alcohol problems in college students. We recruited a diverse sample of urban college students (N = 203, Mage = 21.5 years, SD = 5.5; 79.5% female; 56.8% White, 28.1% Black, .9% Asian, 9.8% Multiracial) who endorsed both alcohol use and PTS symptoms. Reward deprivation (lack of access to, and ability to, experience reward) was related to alcohol problems, and a lack of access to reward was related to PTS symptoms. Furthermore, reward access mediated the relation between PTS symptoms and alcohol problems and craving, after controlling for alcohol use, age, gender, and race. These data provide preliminary support for behavioral economic models of alcohol comorbidity and suggest that treatments for combined PTS and alcohol misuse should attempt to reduce barriers to accessing natural rewards.