BACKGROUND AND OBJECTIVES: People with higher weight are at greater risk of experiencing loneliness and mental ill-health, in part due to challenging social networks that can be unsupportive of efforts to engage in positive health behaviours and a source of weight-based stigma and discrimination. Targeting this issue is a manualised intervention, Groups 4 Health (G4H), that helps people to optimise social connectedness and group-based belonging for effective support to reduce loneliness and mental ill-health. We evaluated the efficacy of this program for people with higher weight. DESIGN AND PARTICIPANTS: We conducted a matched controlled trial (non-randomised) with 98 Australian adults with a body mass index (BMI) ≥ 30 kg/m (n = 49 in the intervention group). Retention rates at T2 were comparable to previous trials: intervention group n = 33 (67.3%); matched control group n = 36 (73.5%). INTERVENTION AND OUTCOMES: Loneliness, well-being, weight-related social support, depression, and eating disorder symptoms were assessed pre-intervention (T0), post-intervention (T1), and at 4-month follow-up (T2). RESULTS: Among the intervention group, loneliness (d = -0.66, p < 0.001), depression (d = -0.58, p < 0.001), and eating disorder symptoms (d = -0.77, p < 0.001) all significantly decreased from T0 to T2. Similarly, well-being (d = 0.80, p < 0.001) and experiences of effective weight-related social support (d = 0.68, p < 0.001) significantly increased from T0 to T2. These positive changes were not observed in the matched control group. CONCLUSIONS: The findings provide strong preliminary support for the efficacy of G4H among people with higher weight to address loneliness and challenging social networks, which pose key psychosocial barriers to health. PATIENT AND PUBLIC CONTRIBUTION: The Groups 4 Health program has previously undergone a published codesign and consumer feedback process. The materials for this study were co-produced with a member of the research team with lived experience to ensure that the content was non-stigmatising and relevant to the population of the study. The research team member is a representative of a consumer advocacy association, and contributed to the study design, data collection, interpretation of results, and manuscript revisions.