BACKGROUND:Depression is a common comorbidity in type 2 diabetes. Studies have consistently shown that major depression is associated with decreased diabetic self-care behaviour. People with subthreshold depression experience greater functional impairment, have a poorer quality of life and use health services more than those without depressive symptoms. Although subthreshold depression impacts self-care behaviour, the relationship between subthreshold depression and diabetes self-care behaviour has not been systematically reviewed. The objective of this systematic review is to determine the association between subthreshold depression and self-care behaviour in adults with type 2 diabetes. METHODS:This protocol will follow the guideline of the Preferred Reporting Items for Systematic Review and Meta-Analysis Protocols 2015 (PRISMA-P-2015). A systematic search of literature will be conducted for observational studies reporting the association between subthreshold depression and self-care behaviour in adults aged 18 years or over and diagnosed with type 2 diabetes. Electronic databases including MEDLINE, EMBASE, PsycINFO, Emcare and CINAHL will be searched using predefined search terms. Title and abstract, full-text screening and data extraction of identified articles will be done by two reviewers independently. Discrepancies will be resolved by a third author. The methodological quality of the included studies will be assessed using The Joanna Briggs Institute (JBI) risk of bias tools. The review results will be presented in the form of narrative synthesis, and if sufficient studies are available and variability among the studies is low, a random effects meta-analysis will be done to quantify the result. DISCUSSION:This review will synthesise evidence on the association between subthreshold depression and self-care behaviour in type 2 diabetic adults. The findings will be useful to researchers and policymakers to determine the most effective approach to overall diabetes management. The review will also identify research gaps in the current literature and provide direction for future research in this area of study. SYSTEMATIC REVIEW REGISTRATION:PROSPERO CRD42018116373.