High social anxiety is related to marijuana problems, yet the nature of this relation remains unclear. We examined relations between marijuana effect expectancies, social anxiety, and marijuana among undergraduates (N=337). Social anxiety was related positively to Negative Expectancies and negatively to Tension Reduction Expectancies. Among socially anxious individuals, greater belief that marijuana produces Cognitive and Behavioral Impairment was associated with greater marijuana use rates. Negative Expectancies mediated the social anxiety-marijuana problems link. These data provide new insight into problematic marijuana use among this high-risk group.