Abstract Background The antiretroviral therapy (ART) adherence club (AC) differentiated service delivery model, where clinically stable ART patients receive their ART refills and psychosocial support in groups has supported clinically stable patientsâ retention and viral suppression. Patients and health systems could benefit further by reducing visit frequency and increasing ART refills. We designed a cluster-randomized control trial comparing standard of care (SoC) ACs and six-month ART refill (Intervention) ACs in a large primary care facility in Khayelitsha, South Africa. Methods Existing ACs were randomized to either the control (SOC ACs) or intervention (Intervention ACs) arm. SoC ACs meet five times annually, receiving two-month ART refills with a four-month ART refill over year-end. Blood is drawn at the AC visit ahead of the clinical assessment visit. Intervention ACs meet twice annually receiving six-month ART refills, with a third individual visit for routine blood collection anytime two-four weeks before the annual clinical assessment AC visit. Primary outcomes will be retention in care, annual viral load assessment completion and viral load suppression. (<400copies/mL) after 2 years. Ethics approval has been granted by the University of Cape Town (HREC 652/2016) and the Medecins Sans Frontieres (MSF) Ethics Review Board (#1639). Results will be published in peer-reviewed journals and made widely available through presentations and briefing documents. Discussion Evaluation of an extended ART refill interval in adherence clubs will provide evidence towards novel model adaptions that can be made to further improve convenience for patients and leverage health system efficiencies. Trial registration Registered with the Pan African Clinical Trial Registry: PACTR201810631281009. Registered 11 September 2018.