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Prior work suggests that exposure to graphic health warning posters (GWPs) at retail point-of-sale may increase future cigarette smoking susceptibility in adolescents who are already at risk for future smoking, but not among committed never-smokers. However, little is known about what psychological mechanisms may account for this effect of GWPs on at-risk youths. Participants (N = 441) aged 11–17 years were randomized to experimental shopping conditions in a life-sized model convenience store, in which GWPs were absent (“status quo”; n = 107) or visibly displayed near the check-out area (n = 334). Participants completed pre- and post- “shopping” measures of future smoking susceptibility, descriptive and injunctive smoking norms, and perceived harms of smoking. A series of linear regression analyses assessed whether norms and harms differentially mediated the effect of experimental condition on future smoking susceptibility in committed never smokers compared with at-risk youths. Tests showed evidence for mediated moderation of the effect of GWP exposure on future smoking susceptibility, such that changes in injunctive norms (i.e., greater perceived social disapproval)—but not descriptive norms or perceived smoking harms—partially accounted for the effect of GWPs on smoking susceptibility in at-risk youths (average causal mediation effect: B = 0.51 [0.14–1.22], p = .02), but not among committed never smokers. For adolescents already at risk of future smoking, GWPs increase perceptions of cigarettes as less socially acceptable, which may increase susceptibility to future smoking in this group. Future work should examine reactance to antismoking messaging among youth at risk for future smoking.