AimTo create a programme theory of family engagement in paediatric acute care to explicate the relationships between contexts and mechanisms of family engagement that align with family, direct care providers and healthcare organization outcomes.DesignRealist review and synthesis.Data SourcesPubMed, CINAHL, PsycINFO and Web of Science searches for the 2.5‐year period (July 2019–December 2021) following our 2021 scoping review.Review MethodsFollowing methods described by Pawson and Rycroft‐Malone, we defined the scope of the review, searched for and appraised the evidence, extracted and synthesized study findings and developed a supporting narrative of our results.ResultsOf 316 initial citations, 101 were included in our synthesis of the final programme theory. Contexts included family and direct care provider individualism, and the organizational care philosophy and environment. Mechanisms were family presence, family enactment of a role in the child's care, direct care providers facilitating a family role in the child's care, unit/organizational promotion of a family role, relationship building and mutually beneficial partnerships. Outcomes were largely family‐focussed, with a paucity of organizational outcomes studied. We identified four context–mechanism–outcome configurations.ConclusionThis realist review uncovered underlying contexts and mechanisms between patients, direct care providers and organizations in the family engagement process and key components of a mutually beneficial partnership. Given that successful family engagement requires direct care provider and organizational support, future research should expand beyond family outcomes to include direct care providers, particularly nurses and healthcare organization outcomes.ImpactThe final programme theory of family engagement in paediatric acute care provides a roadmap for clinicians to develop complex interventions to engage families and evaluate their impact. The components of our final programme theory reflect family engagement concepts that have been evolving for decades.Patient or Public ContributionThe team conducting this review included members from the practice setting (JT & KG). In the future, as we and others use this model in practice, we will seek input for refinement from clinicians, patients and caregivers.