Beliefs about medication necessity and concerns predict treatment adherence in people with a wide-array of medical conditions, including HIV infection. However, medication beliefs have not been examined in people dually treated with psychotropic medications and antiretroviral therapy. In the current study, we used a prospective design to investigate the factors associated with adherence to psychotropic medications and antiretrovirals among 123 dually treated persons living with HIV. We used unannounced phone-based pill counts to monitor adherence to psychiatric and antiretroviral medications over a 6-week period. Hierarchical regression models included demographic, health and psychosocial characteristics as predictors of adherence followed by medication necessity and concerns beliefs. Results showed that medication necessity beliefs predicted both antiretroviral and psychiatric medication adherence over and above established predictors of adherence. Medication concerns also predicted psychotropic adherence, but not antiretroviral adherence. These models accounted for 31 and 22 % of the variance in antiretroviral and psychotropic adherence, respectively. Findings suggest that the necessity-concerns medication beliefs framework has utility in understanding adherence to multiple medications and addressing these beliefs should be integrated into adherence interventions.