BACKGROUND: In 2021, more than two-thirds of the world's children lived in a conflict-affected country. In 2022, 13 million Ukrainians were forced to flee their homes after Russia's full-scale invasion. Hope Groups are a 12-session psychosocial, mental health, and parenting support intervention designed to strengthen parents, caregivers, and children affected by war and crisis. The primary objective of this study is to evaluate the effectiveness of Hope Groups among Ukrainians affected by war, compared to a wait-list control group. This protocol describes a promising decentralized intervention delivery model and an innovative research design, which estimates the causal effect of Hope Groups while prioritizing prompt delivery of beneficial services to war-affected participants. METHODS: This protocol describes a pragmatic cluster randomized controlled trial (RCT) among Ukrainians externally displaced, internally displaced within Ukraine, and living at home in war-affected areas. This study consists of 90 clusters with 4-7 participants per cluster, totaling approximately n = 450 participants. Intervention clusters will receive 12-session Hope Groups led by peer facilitators, and control clusters will be wait-listed to receive the intervention after the RCT concludes. Clusters will be matched on the facilitator performing recruitment and intervention delivery. Primary outcomes are caregiver mental health, violence against children, and positive parenting practices. Secondary outcomes include prevention of violence against women and caregiver and child well-being. Outcomes will be based on caregiver report and collected at baseline and endline (1-week post-intervention). Follow-up data will be collected among the intervention group at 6-8 weeks post-intervention, with aims for quasi-experimental follow-ups after 6 and 12 months, pending war circumstances and funding. Analyses will utilize matching techniques, Bayesian interim analyses, and multi-level modeling to estimate the causal effect of Hope Groups in comparison to wait-list controls. DISCUSSION: This study is the first known randomized trial of a psychosocial, mental health, and parenting intervention among Ukrainians affected by war. If results demonstrate effectiveness, Hope Groups hold the potential to be adapted and scaled to other populations affected by war and crisis worldwide. Additionally, methodologies described in this protocol could be utilized in crisis-setting research to simultaneously prioritize the estimation of causal effects and prompt delivery of beneficial interventions to crisis-affected populations. TRIAL REGISTRATION: This trial was registered on Open Science Framework on November 9, 2023. REGISTRATION: OSF.IO/UVJ67 .