BACKGROUND: In the United States and worldwide, there is a significant number of young people acquiring and living with HIV. Antiretroviral therapy (ART) has led to significant reductions in HIV-related illnesses and deaths, allowing young people living with HIV to manage their condition as a chronic disease. Ensuring high levels of ART adherence is vital for treatment success. Despite this, to the best of our knowledge, there are no efficacious behavioral interventions for improving ART adherence and viral suppression among youth in the United States. METHODS: We conducted a multicity, randomized, controlled trial-in Boston, MA/Providence, RI and Chicago, IL-to examine the efficacy of an stepped care, behavioral and technology-based intervention, "Positive STrategies to Enhance Problem-Solving Skills (Positive STEPS)," compared with a standard of care (SOC) control, for improving ART adherence and viral suppression among youth living with HIV ages 16-29 years. Positive STEPS included the following: step 1: TXTXT, an evidence-based, daily 2-way personalized text message reminder to take ART medications; step 2: only participants with <90% adherence anytime between weeks 5 and 12 postrandomization to the Positive STEPS arm would then receive five 50-minute sessions of manualized individual adherence counseling. If participants' adherence remained at ≥90% , then they did not progress to step 2. ART adherence was measured via Wisepill, an electronic medication monitoring device, and self-report. Participants were followed for 12 months and completed biospecimen collection (HIV plasma RNA viral load testing) and a quantitative assessment battery at baseline and at their 4-, 8-, and 12-month follow-up visits. RESULTS: Between March 2018 and March 2023, 123 participants were randomized (Positive STEPS = 63; SOC control = 60). Intention-to-treat analyses showed a significant positive main effect for the Positive STEPS arm, which increased the mean log ART adherence by 18.7% relative to the SOC control at the 4-month visit (coefficient = 0.187, P = 0.021). For the time effect, a significant overall increase in ART adherence across the subsequent follow-up visits was observed, with increased mean log ART adherence by 27.8% ( P < 0.01) at 8 months and 30.1% at 12 months ( P < 0.01), relative to the SOC control. With respect to our viral suppression outcome, the analysis revealed a significant negative main effect for the Positive STEPS arm at 4-months (odds ratio = 0.264, P = 0.023), indicating that the odds of having an unsuppressed virus were 74% lower in the Positive STEPS arm compared with the SOC control; the interaction term revealed that this effect was maintained through the 12-month follow-up time period. CONCLUSION: These findings on the efficacy of Positive STEPS to enhance ART adherence and viral suppression among youth living with HIV represent the first behavioral intervention for youth to show significant and sustained effects on both behavioral (Wisepill and self-report) and biomedical (HIV viral load) outcomes related to ART adherence. The intervention not only demonstrated remarkable efficacy when compared with the SOC control but also showed maintenance of gains over a 12-month period. Effectiveness and implementation science approaches to testing positive STEPS in real-world settings is recommended.