ABSTRACT Treatment services for HIV and OUD have historically been delivered across multiple settings leading to fragmented and uncoordinated care. Models of behavior change suggest that addressing multiple health conditions simultaneously through integrated, evidence-based, interventions has the potential to overcome traditional barriers to optimize engagement and improve clinical outcomes. Consistent with this perspective, numerous studies have documented that receipt of opioid agonist treatment, in the context of HIV care, is associated with ART adherence and decreased HIV viral loads. Recent pharmacological advancements have led to the development of novel long-acting, injectable, (LAI) medications for both HIV (cabotegravir co-administered with rilpivirine) and OUD (extended-release buprenorphine). These therapies have the potential to dramatically improve adherence by eliminating the need for daily pill-taking. Despite the extensive evidence base supporting LAI for both HIV and OUD, and clinical guidelines supporting integrated care provision, currently little is known about 1) the perceived acceptability/feasibility/safety of combining HIV and OUD treatment into a single point of care; b) which populations and within which type of settings these medications may be optimally delivered; and c) what factors may impede or facilitate future implementation of these treatments. Therefore, the goal of the proposed application is to advance integrated treatment for HIV and OUD by conducting formative research with the target population and key stakeholders to develop a clinical protocol to guide the delivery of combined LAI treatment; evaluate the feasibility, acceptability, and safety of this integrated delivery model; and assess factors to optimize the potential for scalability. The study will be accomplished through two primary aims: (1) Develop a clinical protocol to guide the delivery of combined LAI for HIV and OUD by conducting in-depth interviews with prospective patients, clinical content experts, and other key stakeholders. (2) Conduct an open pilot test of the proposed clinical protocol to assess feasibility, acceptability, and safety among patients diagnosed with HIV and OUD. Throughout all phases of the study, we will simultaneously collect information on patient-, provider-, and organizational-level variables to inform future implementation. Findings from this study will be used to develop a R01 application to conduct a fully-powered Hybrid Type 1 Effectiveness-Implementation study design.