Background: For antiretroviral therapy (ART)to work effectively, adherence is very crucial. However, most of the studies done on ART adherence are either on children or on adults. There is limited information on the level of adherence among adolescents. Since the physical, biological and behavioral changes which occur during adolescence could affect ART adherence, we endeavored to assess the level of adherence among adolescents treated at three hospitals in Addis Ababa, Ethiopia. Study Objective: This study was carried out to assess the magnitude of ART adherence among HIV infected adolescents in Addis Ababa, Ethiopia. It also looked into the various sociodemographic, behavioral and clinical factors associated with ART adherence among adolescents. Study Methods: Using a cross-sectional study design, 273 HIV infected adolescents receiving ART were interviewed from three hospitals in Addis Ababa. We used a structured questionnaire to measure adherence levels using patient self-reports among randomly selected adolescents aged 13-19 years. Data was entered using Epi Info version 3.5.4 and exported to SPSS version 21 for analysis. Bivariate and multivariate methods were used for analysis. Results: We interviewed 273 adolescents aged 13 to 19 years, and 144 (52.7%)of the participants were female. Their mean age was 15.42 years (SD± 1.75). The self-reported adherence rate of the respondents was 79.1% (216/273). Most (96%)were on first line antiretroviral drugs. The median duration on ART was 7 years (IQR, 3). On Bivariate analysis; WHO stage, being on Cotrimoxazole Prophylactic Therapy (CPT), marital and living status of the parent, whether parent was on ART and having special instructions for ART medications were associated with optimum adherence. However only WHO stage IV (adjusted OR, 12.874 95% CI, 2.079-79.706), being on CPT (adjusted OR, 0.339 CI, 0.124-0.97)and adolescents with widowed parent (Adjusted OR, 0.087 with CI, 0.021-0.359) were significantly associated with optimum adolescent ART adherence. Conclusions: The level of self-reported ART adherence among HIV infected adolescents at the three hospitals was below the recommended threshold. Being in advanced WHO clinical stage was associated with better adherence levels, but history of taking CPT was associated with poorer adherence. Earlier presentation of adolescents to care should be encouraged, but more targeted adherence support should be planned for those who present at early stage of their illness.