BackgroundHuman Immunodeficiency Virus (HIV) significantly affects adolescents globally, with the sub-Saharan Africa (SSA) reporting a high burden of the disease. HIV testing, treatment, and retention to care are low among adolescents. We conducted a mixed-method systematic review to assess anti-retroviral therapy (ART) adherence; barriers and facilitators to ART adherence and ART outcomes among adolescents living with HIV and on ART in sub-Saharan Africa.MethodsWe conducted searches in four scientific databases for studies conducted between 2010 and March 2022 to identify relevant primary studies. Studies were screened against inclusion criteria and assessed for quality, and data was extracted. Meta-analysis of rates and odd ratios was used to plot the quantitative studies and meta-synthesis summarized the evidence from qualitative studies.ResultsA total of 10 431 studies were identified and screened against the inclusion/ exclusion criteria. Sixty-six studies met the inclusion criteria (41 quantitative, 16 qualitative, and 9 mixed-methods study designs). Fifty-three thousand two hundred and seventeen (53 217) adolescents (52 319 in quantitative studies and 899 in qualitative studies) were included in the review. Thirteen support focused interventions for improved ART adherence were identified from quantitative studies. The plotted results from the meta-analysis found an ART adherence rate of 65% (95%CI 56–74), viral load suppression was 55% (95%CI 46–64), un-suppressed viral load rate of 41% (95%CI 32–50), and loss to follow up of 17% (95%CI 10–24) among adolescents. Meta-synthesis found six themes of barriers to ART (social, patient-based, economic, health system-based, therapy-based, and cultural barriers) in both the qualitative and quantitative studies, and three themes of facilitators to ART were also identified (social support, counselling, and ART education and secrecy or confidentiality) from qualitative studies.ConclusionART adherence remains low among adolescents in SSA despite multiple interventions implemented to improve ART adherence. The low adherence rate may hinder the attainment of the UNAIDS 2030 targets. Additionally, various barriers to ART adherence due to lack of support have been reported among this age group. However, interventions aimed at improving social support, educating, and counselling adolescents may improve and sustain ART adherence.Trial registrationSystematic review registration:PROSPEROCRD42021284891.