Abstract Background Eliminating hepatitis C virus (HCV) will require effective treatment delivery to persons with substance use disorders (SUDs). We evaluated the relationship between ledipasvir/sofosbuvir treatment persistence (receiving 84 tablets), adherence, and sustained virologic response (SVR) in persons with human immunodeficiency virus (HIV)/HCV coinfection. Methods Of the 144 participants with HIV/HCV and SUDs, 110 initiated a 12-week treatment course under 1 of 3 conditions (usual care, peer mentors, and cash incentives). We used self-report, pharmacy pill counts, and expected date of refill to examine adherence. Persistent participants were categorized as high adherence (taking ≥90% of doses) or low adherence (taking <90% of doses). Results Most participants persisted on treatment after initiation (n = 105), with 95% (n = 100) achieving SVR. One third (34%) of participants had moderate/heavy alcohol use by the biomarker phosphatidylethanol (Peth ≥50 ng/mL), and 44% had urine toxicology positive for cocaine or heroin at enrollment. The proportion of persons with high adherence was 72% (n = 76), and the proportion of persons with low adherence was 28%. Although low adherence was associated with moderate/heavy alcohol use by PEth (relative risk = 2.77; 95% confidence interval, 1.50–5.12), SVR did not vary according to adherence (P = .702), and most participants (97%) with low adherence achieved SVR. Conclusions Treatment persistence led to high SVR rates among persons with HIV/HCV, despite imperfect adherence and SUDs. In this RCT of persons with HIV, HCV, and substance use disorders, 76% initiated a 12-week treatment course for HCV. Participants with high adherence (≥90% of doses) and low adherence (<90% of doses) had high rates of sustained virologic response.