Introduction Self-regulation is a modifiable protective factor for lifespan mental and physical health outcomes. Early caregiver-mediated interventions to promote infant and child regulatory outcomes prevent long-term developmental, emotional and behavioural difficulties and improve outcomes such as school readiness, educational achievement and economic success. To harness the population health promise of these programmes, there is a need for more nuanced understanding of the impact of these interventions. The aim of this realist review is to understand how, why, under which circumstances and for whom, early caregiver-mediated interventions improve infant and child self-regulation. The research questions guiding this review were based on consultation with families and community organisations that provide early childhood and family services. Methods and analysis Realist reviews take a theory-driven and iterative approach to evidence synthesis, structured around continuous refinement of a programme theory. Programme theories specify context-mechanism-outcome configurations to explain what works, for whom, under which circumstances and how. Our initial programme theory is based on prior work in this field and will be refined through the review process. A working group, comprising service users, community organisation representatives, representatives from specific populations, clinicians and review team members will guide the evidence synthesis and interpretation, as well as the development and dissemination of recommendations based on the findings of the review. The review will involve searching: (i) electronic databases, (ii) connected papers, articles and citations and (iii) grey literature. Decisions to include evidence will be guided by judgements about their contribution to the programme theory and will be made by the research team, with input from the working group. Evidence synthesis will be reported using the Realist and MEta-narrative Evidence Synthesis: Evolving Standards guidelines. Ethics and dissemination Ethical approval is not required as this is a review. Findings will be disseminated to our working group and through peer-reviewed publications and conference presentations. Review registration number The protocol is registered with Open Science Framework https://osf.io/5ce2z/registrations.