Abstract We will conduct a Hybrid Type I effectiveness-implementation randomized controlled trial of the Project nGage (“nGage”) intervention, an evidence-based, flexible, and tailored intervention that harnesses social support to promote retention in care and viral suppression (VS) among HIV-positive young Black MSM (YBMSM) aged 18- 35. The study will take place in Chicago, IL, and Alabama (AL), two high-burden areas prioritized in the national Ending the HIV Epidemic Plan. Existing efforts to improve Continuum of Care outcomes for YBMSM often rely on newly created network members, e.g., peer navigators, support groups, case managers. Often missing from these approaches is a focused attempt to harness organic social network supports, i.e., those people who already offer critical forms of emotional, informational, and instrumental support. In contrast, the nGage intervention was developed to identify, activate, and harness organic social network support for YBMSM living with HIV. The intervention uses (1) social network visualization and theory to help men identify a Support Confidant (SC) to engage in care; (2) the Information-Motivation-Behavioral Skills Model targeted at the SC to activate and maintain dyadic social support; (3) a linked social support model to target the drivers of retention in care and VS. Content is delivered via a single face-to-face session and quarterly mini-boosters. nGage's flexibility ensures that SCs are selected based on their supportive function rather than their role. In a pilot RCT in Chicago, we demonstrated feasibility, acceptability, and efficacy. To now test effectiveness, N=600 YBMSM living with HIV in Chicago and AL will be randomized to receive nGage (n=300) or treatment as usual (TAU) (n=300). We also will enroll 300 SCs. At 12-months post-intervention, we will re-randomize nGage dyads to continue receiving quarterly mini-boosters (Sustained nGage: n=150) or return to TAU (n=150). Data collection at baseline, 12, and 24 months will include surveys and electronic medical record (EMR) data. To study implementation, we will use the Consolidated Framework for Implementation Research (CFIR) as the determinants framework and RE-AIM as the evaluation framework. The specific aims are to: (Aim 1) Evaluate the (a) effectiveness of nGage vs. TAU over 12 months in N=600 YBMSM aged 18-35 and (b) value of continuing nGage over another 12 months (Sustained nGage). The primary outcomes are retention in care and VS, as measured by EMR data; (Aim 2) Examine if intervention effects (a) vary between Chicago and AL, (b) are mediated by changes in the Index's motivational readiness, stigma expectancies, and self-efficacy, and (c) are moderated by Index's mental health and substance use; and (Aim 3) Evaluate the implementation of nGage using the CFIR and the RE-AIM framework. Guided by the CFIR, we will conduct surveys and focus groups with key stakeholders to assess the inner and outer settings, implementer and intervention ch...