Multimorbidity, characterised by the coexistence of two or more chronic conditions, represents a growing challenge for health systems, adversely affecting quality of life, self-care, treatment adherence and service utilisation. Within the context of self-regulation in health, the Common-Sense Model of Self-Regulation of Health and Illness (CSM) proposes that illness perceptions—such as beliefs about causes, control, consequences, timeline and illness identity—influence health behaviours. Despite consolidated evidence in single-disease contexts, little is known about how these perceptions operate when multiple conditions coexist, particularly due to the need to integrate potentially conflicting representations. To date, no systematic reviews have synthesised these relationships specifically in populations with multimorbidity. This protocol describes the methods for a systematic review aimed at examining how illness perceptions are associated with treatment adherence, self-care and indicators of chronic condition management in adults with multimorbidity.

This systematic review will be reported in accordance with the Preferred Reporting Items for Systematic Review and Meta-Analysis Protocols (PRISMA-P) guidelines. Observational studies involving adults (≥18 years) with multimorbidity, which assess illness perceptions using validated instruments and report outcomes related to treatment adherence, self-care or self-management behaviours, will be included. The search will be conducted in the PubMed, Scopus, Web of Science, Embase, CINAHL, PsycINFO and LILACS databases, using combinations of descriptors related to illness perception, multimorbidity and health behaviours. Two independent reviewers will conduct study selection, data extraction and methodological quality assessment. The risk of bias will be assessed using the Newcastle-Ottawa Scale, and the overall quality of the evidence will be evaluated using the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach. Where appropriate, meta-analysis will be conducted using measures of association (eg, ORs, relative risk or HRs); otherwise, a structured narrative synthesis will be performed. Subgroup analyses will be conducted when data are available.

As it uses exclusively secondary and published data, this review does not require approval by a research ethics committee. The results will be submitted for publication in a peer-reviewed journal and disseminated at relevant scientific conferences.

CRD420251266292.