Objective The aim of the study was to assess whether a simple, routinely available measure of antiretroviral therapy (ART) adherence predicts viral rebound at the next HIV viral load (VL) measurement in virally suppressed patients. Methods The analysis was performed on the Royal Free HIV Cohort, London, UK. Each ‘drug coverage–viral load episode’ (DCVL episode) was defined as a 6‐month period immediately prior to a VL ≤50 HIV‐1 RNA copies/mL (time‐zero), during which the patient had been continuously on HAART, with all measured VLs ≤50 copies/mL. The next VL after time‐zero was used to assess whether VL rebound (defined as >200 copies/mL) had occurred. Drug coverage, our measure of adherence, was calculated as the proportion of days in the 6‐month period covered by a valid prescription for at least three antiretroviral drugs. Results A total of 376 (2.4%) VL rebounds occurred in 15 660 DCVL episodes among 1632 patients. Drug coverage was 100% for 32% of episodes, 95–99% for 16% of episodes and ≤60% for 10% of episodes. The risk ratio of rebound associated with a 10% increase in drug coverage, adjusted for potential confounding variables, was 0.93 (95% confidence interval 0.88–0.98). Conclusions Antiretroviral drug coverage assessed at the time of VL measurement in patients with undetectable VL is potentially clinically useful for predicting VL rebound at the next VL measurement.