Introduction: Supported housing−which offers permanent, community-based housing and supportive services−effectively addresses homelessness. Yet, many persons with serious mental illness (SMI) struggle to retain housing in these programs. Social skills−which facilitate social interactions and instrumental tasks−predict premature exits from supported housing. Though social skills training effectively improves social skills and functioning for persons with SMI, this intervention is essentially absent from supported housing initiatives. Methods: This study will use literature review, key informant interviews and an expert panel to adapt social skills training for supported housing, aiming to improve housing retention among SMI persons in these programs. In the Department of Housing and Urban Development-VA Supported Housing program (HUD-VASH) at VA Greater Los Angeles, we will conduct an effectiveness-implementation hybrid type I trial of the adapted intervention. Baseline and 6-month assessments (measuring social skills, mental health status, healthcare utilization, social networks, money management, and housing outcomes) will be conducted with intervention participants (n = 30) and a usual care control group (n = 20). We will use the generalized linear mixed model (GLMM) to compare change over 6 months between groups, capturing the intervention's effectiveness on factors strongly associated with housing retention. Qualitative data and surveys with staff, leadership, and participants will gather data on factors relevant to the intervention's future implementation in routine care. Discussion: Few effective psychosocial interventions for persons with SMI have been adapted or studied in supported housing initiatives. To our knowledge, this is the first study to adapt and study the effectiveness and implementation of social skills training in supported housing programs.