Background <p>The lack of a standardized and theoretically grounded instrument to assess psychosocial determinants of sarcopenia prevention represents a critical gap in public health. This protocol describes the development and validation of the Knowledge, Belief, and Behaviour Questionnaire on Sarcopenia for Older Adults (KBBQ-SOA).</p> Methods <p>Guided by the Consensus-based Standards for the selection of Health Measurement Instruments (COSMIN) and informed by the Knowledge-Belief-Behaviour framework, the study will be conducted in four sequential stages: (1) item generation and construct clarification through literature review, expert consultation, and semi-structured interviews; (2) content validation using a Delphi expert panel and cognitive interviews; (3) psychometric evaluation in a multi-site sample of 300–500 community-dwelling older adults across four diverse regions of China; and (4) assessment of interpretability and feasibility, including a preliminary evaluation of cross-cultural applicability.</p> Outcomes <p>This paper presents the study design and methodology; no results are reported. </p> Conclusion <p>The KBBQ-SOA is expected to be a rigorously validated, multi-dimensional instrument for assessing sarcopenia-related knowledge, beliefs, and behaviours. It may support the development and evaluation of targeted health education and behavioural interventions in older adult populations.</p> Trial registration <p><a href="https://doi.org/10.17605/OSF.IO/XMJY4">https://doi.org/10.17605/OSF.IO/XMJY4</a></p>