Background: Adherence to antiretroviral therapy (ART) is important to counter synergistic effects of HIV and hepatitis C (HCV) in patients living with coinfection. Predictors of ART nonadherence among patients living with HIV/HCV coinfection are not well established. This knowledge would be advantageous for clinicians and behavioral health specialists who provide care to patients living with HIV/HCV coinfection. Objectives: The purpose of this study was to assess prevalence and predictors of ART nonadherence in a sample of patients living with HIV/HCV coinfection who were actively in HIV clinical care. Method: A sample of patients living with HIV/HCV coinfection who received care at a university-affiliated HIV clinic (n = 137) between January 2013 and July 2017 were included in the study. Computerized patient-reported data or outcomes (PROs) and electronic medical record data of these respective patients were collected and analyzed. Binomial logistic regression was used to examine predictors of ART nonadherence. Results: The prevalence of ART nonadherence was 31%. In multivariate analysis, African American ethnicity (OR = 3.28, CI 1.241–8.653, p = 0.017) and a higher number of alcoholic drinks per drinking day (OR = 1.31, CI 1.054–1.639, p = 0.015) were positively associated with ART nonadherence. Conclusions: Behavioral health providers are encouraged to incorporate alcohol use reduce interventions in HIV clinical settings to reduce ART nonadherence among patients living with HIV/HCV coinfection. Additionally, public health professionals and researchers, and clinicians are encouraged to use inductive methods to discover why ART nonadherence disproportionately impacts African American patients living with HIV/HCV coinfection and to develop approaches that are sensitive to those respective barriers.