BACKGROUND: Public and Patient Involvement (PPI) in mental health research is increasingly recognised as a moral and ethical imperative, necessary to increase the relevance and effectiveness of translation of research findings. Despite policy mandates and growing evidence of its benefits, PPI implementation in mental health research remains inconsistent. Little attention has been given to the state of scientific knowledge on PPI capacity strengthening in mental health research that can support more meaningful implementation. The aims of this scoping review are to: describe the content, implementation process, and theoretical underpinnings of PPI capacity-strengthening initiatives in mental health research; identify quantitative outcome measures and outcomes used to evaluate the initiatives' impact on PPI contributors, research processes, and policy; and map barriers and enablers to the initiatives' implementation.

METHODS: This scoping review will follow JBI and PRISMA-ScR guidelines. Sources will include peer-reviewed articles, grey literature, and organisational materials describing training or skill-building initiatives for adult PPI contributors in mental health research. Searches will be conducted in MEDLINE, Embase, PsycINFO, and CINAHL, supplemented by hand-searching, targeted internet searches, and stakeholder consultation. Data extraction will capture descriptive details, initiative content, outcomes, and contextual factors, with barriers and enablers categorised according to the Consolidated Framework for Implementation Research (CFIR) domains.

CONCLUSION: This review will provide the first comprehensive synthesis of capacity-strengthening initiatives for PPI contributors in mental health research. Findings will inform the development of a co-designed blueprint for capacity-strengthening for PPI contributors, and progress broader efforts to embed lived experience expertise and general public perspectives equitably within mental health research systems.