Existing efforts to understand school-based child sexual abuse (CSA) prevention programs mainly focus on the effectiveness of these programs in increasing participants' CSA knowledge and self-protective skills. There are currently no reviews addressing the underpinning pathways leading to these outcomes. In order to increase our understanding about the underpinning causal and contextual factors and inform the further development of school-based CSA prevention programs, a realist review was conducted to synthesize existing evidence from a broad range of data. An iterative search of electronic databases and grey literature was conducted, supplemented with citation tracking to locate relevant literature. For quantitative evidence, we considered evaluation studies that focused on students aged 5-18 years, who were enrolled in primary or secondary schools; for other types/formats of studies/documents, no population restrictions were applied. We included school-based CSA prevention programs that focused on improving knowledge of CSA or self-protective skills. Outcomes of interest included knowledge of CSA or self-protective skills. We did not apply methodological filters in terms of the types of studies to be included. Thematic content analysis was conducted to synthesize data. Sixty-two studies were included. Five themes and five overarching Context-Strategy-Mechanism-Outcome configurations (CSMOs) that contributed to the success of school-based CSA interventions were identified, including tailoring programs to participants' cognitive developmental levels, repeated exposure of key concepts and skills, utilization of interactive delivery methods and positive feedback, delivery of positive information and application of the 'train-the-trainer' model. Findings from this realist review provide insights into the underlying program theory of school-based CSA prevention programs, which can aid in the development and implementation of these programs in the future.