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Introduction To preserve the validity and reliability of health instruments like patient-reported outcome measures across linguistic and cultural contexts, translation, cross-cultural adaptation and psychometric validation must be conducted using standardised, rigorous methodologies. Although international standards proposed by the International Society for Pharmacoeconomics and Outcomes Research (ISPOR) and the COnsensus-based Standards for the selection of health Measurement INstruments (COSMIN) are available, adherence to these guidelines in Bangla-language instrument studies has not been systematically evaluated. This systematic review aims to identify, synthesise and critically appraise Bangla-language translation, cross-cultural adaptation and psychometric validation studies of health instruments for adherence to recommended methodological approaches, translation and cross-cultural adaptation guidelines and psychometric properties.Methods and analysis In this study, we will search three databases, including Ovid MEDLINE, PubMed and EMBASE via Ovid, from 1 February 2026 to 1 March 2026 to retrieve published articles on translation, adaptation and psychometric validation of health-related instruments into Bangla. Two reviewers will independently conduct title and abstract screening, full-text eligibility assessment, data extraction, evaluation of translation and cross-cultural adaptation quality and assessment of psychometric properties. The certainty of evidence will be graded. Disagreements will be resolved by consensus, with the consultation of a third reviewer when necessary. Methodological quality and psychometric properties will be assessed based on the COSMIN Risk of Bias (RoB) checklist and COSMIN standards. Evaluation of translation and cross-cultural adaptation will be based on the extended ISPOR guideline. COSMIN-Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach will be used to rate the certainty of evidence.Ethics and dissemination This study is a systematic review protocol, exclusively on published literature and publicly available data, and does not involve direct interaction with human participants or the collection of individual participant-level data. Therefore, ethical approval is not applicable. We expect to share our findings through national and/or international platforms. Moreover, the results will be published in a peer-reviewed journal. We anticipate that the result of this study will contribute to recommending a standardised roadmap for researchers, especially for conducting translation, validation and cross-cultural adaptation studies of health-related instruments in Bangla.PROSPERO registration number CRD420251250949.