Appearance concerns are common among HIV-infected individuals, and previous cross-sectional and longitudinal data indicate that these concerns are associated with antiretroviral therapy (ART) non-adherence. However, to date, no known prospective data have explored the mechanism behind this relationship. Thus, the aim of the current study was to test depression severity as a prospective mediator of the relationship between appearance concerns and ART non-adherence in HIV-infected individuals with a history of injection drug use (IDU). Participants were 89 HIV-infected individuals with a history of IDU who participated in a prospective, randomized controlled trial of cognitive behavioral therapy for depression and medication adherence. Clinician-administered measures of depression severity and appearance concerns, along with electronic monitoring of ART non-adherence were included. Data were analyzed using longitudinal linear mixed-level modeling, and mediation was tested via the Monte Carlo Method of Assessing Mediation. Appearance concerns were predictive of depression severity, γ=.31, SE=.076, 95% CI [.16, .46], t=4.1, p=.0001, and depression severity was predictive of ART non-adherence, γ=3.3, SE=1.3, 95% CI [.8, 5.8], t=2.6, p=.01. The effect of appearance concerns on ART non-adherence, however, was significantly mediated by depression severity, γ=1.02, 95% CI [.21, 2.1]. Appearance concerns are associated with depression severity, which in turn is associated with ART non-adherence. Integrative interventions addressing appearance concerns, depression and ART adherence are needed, as this is one potential pathway towards worse health outcomes in HIV-infected individuals.