Background:The increasing number of people who experience mental disorders is a global problem. People with mental disorders have high rates of co-morbidity and significantly poorer oral health outcomes than the general public. However, their oral health remains largely a hidden and neglected issue. A complex range of factors impact the oral health of this group. These include anxiety and dental phobia, dietary habits, including the heavy consumption of sugary drinks, substance misuse of tobacco, alcohol, and/or psychostimulants, the adverse orofacial side effects of anti-psychotic and anti-depression medications, and financial, geographic, and social barriers to accessing oral health care. Methods:The aim of this realist systematic review is to (a) identify and synthesise evidence that explores oral health interventions for people living with mental disorders; (b) explore the context and mechanisms that have contributed to the success of interventions or the barriers and challenges; (c) produce program theories on causal, contextual and mechanistic factors to facilitate outcomes and (d) produce recommendations and guidelines to guide future oral health interventions for people with mental disorders at both the policy and practice level. Using a five-step process, that incorporates primary data collection from key stakeholders, a beginning theoretical framework will be developed to describe contextual and mechanistic factors and how they might impact on the success or failure of oral health interventions for people with mental disorders. Key database searches will be conducted, with data extraction focused on the factors that might have impacted on intervention implementation and outcomes. Quality appraisal of studies will occur, and the theoretical framework will be populated with extracted data. Stakeholder input will support the development and refinement of a theory on oral health interventions for people with mental disorders. Discussion:This will be the first review to take a realist approach to explore the broad scope of causal factors that impact on the success or failure of oral health interventions for people with mental disorders. The approach includes extensive stakeholder engagement and will advance realist systematic review methodology. Review outcomes will be important in guiding policy and practice to ensure oral health interventions better meet the needs of people with mental disorders.Systematic review registration This review protocol is registered with PROSPERO (Number) 155969.