Negative attitudes toward HIV medications may restrict utilization of antiretroviral therapy (ART) in Indonesian prisons where many people living with HIV (PLH) are diagnosed and first offered ART. This mixed-method study examines the influence of medication attitudes on ART utilization among HIV-infected Indonesian prisoners. Randomly-selected HIV-infected male prisoners (n = 102) completed face-to-face in-depth interviews and structured surveys assessing ART attitudes. Results show that although half of participants utilized ART, a quarter of those meeting ART eligibility guidelines did not. Participants not utilizing ART endorsed greater concerns about ART efficacy, safety, and adverse effects, and more certainty that ART should be deferred in PLH who feel healthy. In multivariate analyses, ART utilization was independently associated with more positive ART attitudes (AOR = 1.09, 95 % CI 1.03-1.16, p = 0.002) and higher internalized HIV stigma (AOR = 1.03, 95 % CI 1.00-1.07, p = 0.016). Social marketing of ART is needed to counteract negative ART attitudes that limit ART utilization among Indonesian prisoners.