Background: Custodial grandparents often protect against childhood traumaand promote resilience in their grandchildren. In this role, grandparents face many challenges and may lack confidence in their parenting abilities. Increasing grandparent self‐efficacy may help promote grandfamily resilience. Yet few interventions exist to help grandparents adapt to challenges faced as primary parenting figures for grandchildren. Objective: The purpose of this study was to examine the preliminary efficacy of a strengths‐based intervention for grandparents raising grandchildren and explore whether it would improve grandparent self‐efficacy. Method: Grandparents providing primary care to grandchildren participated in a 6‐week intervention, single‐group, pretest–posttest design. Self‐efficacy was assessed at baseline (N = 149), post‐intervention (n = 104), and 6 months (n = 55). A linear mixed effects analysis was conducted to test the effect of the intervention on grandparent self‐efficacy. Results: The intervention was associated with small improvements in grandparents’ self‐efficacy, after controlling for demographic variables and perceived social support. These improvements were sustained over time. Conclusion: Our intervention addresses the complex needs of grandparents raising grandchildren and has a positive effect on their self‐efficacy. This study provides support for the feasibility of strengths‐based intervention programs for grandfamilies. Implications: Programs that foster effective strategies to enhance self‐efficacy are important for the well‐being of grandfamilies.