Promoting Social Connection to Prevent Late-Life Suicide Older adults have high rates of suicide and projections indicate subsequent cohorts will usher in even higher rates. We can expect an increase in suicide deaths among older adults in coming decades. However, there are no evidence-based interventions to prevent suicide deaths in later life, and scant data support strategies to prevent suicide deaths at any age. Social disconnection is a promising intervention target for late-life suicide prevention, but it is not known whether targeting social connection is an effective strategy to prevent suicide in later life. The objective of this R61/R33 proposal, in line with NIMH priorities of reducing suicide and understanding mechanisms, is to test whether a behavioral intervention—Social Engage Coaching (S-ENG)—reduces suicide risk in later life (clinical outcome) via social connection (target mechanism). We propose a two-phase experimental therapeutics project with multimodal assessment of objective and subjective indicators of social connection (via smartphone assessments). The R61 is a single-arm trial of S-ENG (n=30) with 1 week of smartphone-based target assessment at baseline, 8-weeks, and 16-weeks. The R33 is an RCT (S-ENG vs. eCAU; n=120) with longer duration follow-up to test clinical impact (20-weeks). Subjects are lonely older adults in Senior Living Communities (SLCs) who endorse suicide ideation. Study conditions are S-ENG (10 individual coaching sessions) and enhanced usual-care in SLCs (eCAU; exposure to SLC milieu with geriatrician medical directors). Our first aim for the R61 phase is to test target engagement (social connection). We hypothesize that S-ENG will be associated with increases in four indices of social connection. Our go/no-go criteria will determine if we move to the R33 phase: At least two indices of social connection must evidence clinically meaningful improvement—one objective and one subjective. If go/no-go criteria are met, we will address two aims for the R33 phase. First, to confirm target engagement in a randomized trial. We hypothesize that S-ENG (vs. eCAU) will be associated with greater increases in social connection. Second, to examine clinical and functional indices of reduced suicide risk. We hypothesize that S-ENG (vs. eCAU) will be associated with greater improvement in suicide ideation and quality of life at 20-week follow-up and improvement in social connection at 16-weeks will be associated with reduction in indices of suicide risk. A confirmatory efficacy trial will follow this study. Promoting social connection to prevent suicide is an under- studied but promising strategy to address mental health and well-being in later life.