Supporting Antiretroviral Therapy (ART) adherence is crucial in achieving, improving and maintaining viral suppression among people living with HIV. Hitherto, support systems have shown mixed intervention outcomes across the African region. Whereas an 85-95% level of ART adherence is critical to the success of treatment and maintaining viral suppression, the viral load suppression rate in Kenya is still suboptimal at 70%. Uasin Gishu County, previously a low HIV burden county, is currently classified among the top ten counties that contributes 53% of Kenya’s new HIV infections. There was a need to determine the ART adherence and viral suppression level among adolescents living with HIV (ALHIV) in this County and to evaluate which of the support interventions in place are being implemented and their impact on ART adherence. The current study assessed the level of adherence to ART and viral load (VL) suppression; described the support systems for antiretroviral therapy adherence, and; determined the association between the support systems and ART adherence among ALHIV in Uasin Gishu County, Kenya. Using a descriptive cross-sectional survey design with a quantitative approach, data was collected from 277 respondents using a structured questionnaire, between February and April 2023. Viral suppression data was obtained from the AMPATH Medical Records System (AMRS). Respondents were identified using systematic random sampling technique, as they came for routine clinic visits at a large HIV comprehensive care clinic namely Rafiki Center and Module 4 both at Academic Model Providing Access to Healthcare (AMPATH) and domiciled within Moi Teaching and Referral Hospital. Rafiki Center and Module 4 clinics offer comprehensive care to over 800 ALHIV. Data was analyzed descriptively to determine frequency distributions and percentages. Bivariate and robust multivariate logistic regression analyses was used to examine variable associations. Of the 277 adolescents, 263 had complete data following data cleaning. Of the 263 adolescents, 201 (76.4%) were categorized as adherent to ART and 225 (85.6%) were virally suppressed. Of the hospital-based support systems components, individual counseling (OR=2.93, 95%CI= 1.15-7.46, p= 0.024) and receiving reminder messages/calls from the health facility (OR=2.72, 95%CI=1.21- 6.09, p= 0.015) were associated with higher ART adherence compared to those who did not receive. Conversely, counseling by clinical psychologists was linked to lower adherence (OR=0.45, 95%CI=0.25- 0.81, p-value= 0.007) as compared to those not counseled by them. Regarding household-based support components, participants who freely discussed their HIV status with caregivers/parents (OR=3.13, 95%CI=1.56-6.29, p-value= 0.001); received reminders from caregivers to take ART (OR=1.99, 95%CI=1.03-3.83, p-value=0.039); had family members aware of their HIV status (OR=2.51, 95%CI=1.27-4.96, p-value=0.008) or; received financial support for travel to the facility (OR=2.37, 95%CI=1.19-4.71, p-value=0.014) were more likely to adhere to ART. The study concluded that there was a relatively high level of ART adherence and viral suppression. The hospital- and household-based support systems improved ART adherence differentially through individual counseling, use of reminders and within-family interpersonal support. Despite the programmatic efforts, approximately a quarter of the adolescents do not adhere to ART. The information obtained from this study is significant for guiding the design of policy and programmatic interventions on implementation mechanisms of ART adherence support systems. This study recommends that program interventions targeting improvement of provider as well as family capacities and resources to be put in place, which could further enhance the impact of hospital and household support systems on ART adherence among ALHIV.