Background: Social isolation is a global public health threat and a negative social determinant of health (SDoH) that affects 1 in 5 adults in the U.S. and contributes to $6.7 billion in annual healthcare spending. Significance: Affecting roughly 43% of Veterans, social isolation is highly prevalent among Veterans, particularly those with a history of mental illness. Social isolation contributes to cardiovascular diseases, dementia, depression, suicidal ideation, and premature death. To date, social isolation remains largely unaddressed as a negative SDoH in healthcare systems. Current efforts to address social isolation are limited by lack of diverse participant samples, rigorous methodologies, and involvement of healthcare systems to systematically assess and reduce social isolation. Innovation and Impact: To address these gaps, the proposed project will test a novel and feasible program to intervene on social isolation among diverse Veteran populations in the VHA healthcare system. We propose to test the effects of the Increasing Veterans’ Social Engagement and Connectedness (CONNECTED) intervention on social isolation among Veterans. CONNECTED uses an innovative approach by integrating two existing evidence-based care models: peer services and patient navigation to address social isolation among Veterans in VHA primary care clinics. CONNECTED involves three key services delivered by peer specialists via telehealth over 8 weeks: 1) peer support, which includes person-centered assessment of factors driving social isolation; 2) psychosocial interventions to address Veterans’ social isolation (e.g., goal setting, supportive therapy, and group-based social engagement activities); and 3) navigation (i.e., connecting Veterans to social resources in the community and the VHA) to help expand their social networks. Because prior social isolation studies have not prioritized inclusion of younger and racially/ethnically diverse samples, we will oversample these groups using stratified random sampling. Specific Aims: We aim for a randomized controlled clinical trial comparing CONNECTED to an attention control group. Aim1: Test the effects of CONNECTED on social isolation at 2-, 4-, and 8-months from baseline compared to the control group; Aim2: Test the effects of CONNECTED on secondary, health-related outcomes; and Aim 3: Conduct formative and pre-implementation evaluations to inform future implementation processes. Methodology: Aims 1 and 2 involve delivering the intervention to (N=264) Veterans in VHA primary care clinics. Data from Aims 1 and 2 will be analyzed using general linear models. In Aim 3, we will describe Veterans (n=20) and providers’ experiences (N=10) with the intervention using semi-structured interviews to identify barriers and facilitators to CONNECTED’s future implementation. We will also survey peers and peer supervisors (n=20) from VISN10 VA facilities (n=13) and interview a subgroup of survey completers (n=8) to evaluate factors t...