BACKGROUND: Social, emotional and behavioural (SEB) problems are among the most common chronic disabilities affecting children growing up in poverty. They also have implications for children's school success as they affect essential social-emotional learning skills such as the ability to comply with rules, regulate emotions and get along with others. These skills are first learnt before kindergarten, in the context of a supportive, responsive and consistent parenting relationship. To date, school-based interventions to improve young children's SEB competence and learning have primarily targeted students and teachers. Yet, parents are central partners in promoting these skills. This study seeks to improve children's SEB competence and kindergarten readiness by strengthening parenting skills and parent engagement in early childhood education during prekindergarten (PreK). This hybrid type 2 effectiveness-implementation trial will rigorously evaluate the effects of an evidence-based parenting programme, the Chicago Parent Program (CPP), in PreK on children's SEB competence, kindergarten readiness, chronic school absenteeism and grade retention in urban and rural schools serving students from low-income families in Maryland. METHODS: Using a cluster randomised design (n=30 schools, 840 parents; >90% low-income), we will examine the effects of CPP offered universally to PreK parents on parenting skills and parent engagement in children's education; children's SEB competence and kindergarten readiness; and chronic absence and grade retention in kindergarten. Schools will be stratified by rural versus urban district, then randomised to CPP or usual practice conditions. Data will be analysed using mixed effects regression models. Using the reach, effectiveness-adoption, implementation, maintenance (RE-AIM) framework and a mixed methods approach, we will assess CPP reach, efficacy, acceptability, adoption, implementation, cost-effectiveness and sustainability when offered in different formats (virtual vs in-person CPP groups) and contexts (urban vs rural). Schools will participate for 2 years with experimental schools offering CPP twice, once in virtual group format and once in an in-person group format (format randomised and counterbalanced). Data will be collected using multiple informants (parents, teachers, district administrative data) and methods (quantitative and qualitative data). Knowledge gained will inform schools in under-resourced urban and rural communities on sustainable, cost-effective strategies for strengthening parent-school connections and improving young children's SEB competence and academic success. ETHICS AND DISSEMINATION: Ethics approval has been granted by Johns Hopkins University School of Medicine (protocol number 00428221) and the Baltimore City Public Schools (protocol number 2024-013). At the conclusion of the study, results will be summarised and shared with parents, teachers, school principals and district leaders for their perspectives on the outcomes. Final reports will be published in scientific journals and presented at professional meetings. TRIAL REGISTRATION NUMBER: NCT06197997.