Background <p>Individuals with mental disorders experience higher morbidity and mortality than the general population, primarily due to co-occurring physical illnesses. Diagnostic overshadowing (DO)—defined as the misattribution of physical symptoms to mental illness—exacerbates health inequities and compromises universal health coverage goals.</p> Subject <p>To advance understanding of the impact of DO, we aim to explore the perspectives of both mental health service users and healthcare professionals.</p> Methods <p>A mixed-methods systematic review was conducted. Comprehensive searches were performed in MEDLINE/PubMed, Web of Science, PsycInfo, and CINAHL up to September 2025. Methodological quality was assessed using the Mixed Methods Appraisal Tool. A convergent integrated approach, following the JBI methodology for mixed-methods systematic reviews and implemented through JBI SUMARI, was employed for synthesis and integration. The review adhered to PRISMA guidelines, and the protocol is registered in PROSPERO (CRD42024523764).</p> Results <p>Seven integrated findings on DO were identified, with frequencies ranging from 5.9% to 52.9%. The most prevalent were environmental facilitators of DO misattribution, clinical practices of premature physical-to-mental attribution, and physical consequences of DO. Factors that promote identification or prevention, stereotypes specifically undermining physical symptom credibility, emotional consequences of poor care, and professional challenges in providing adequate care were less frequent.</p> Conclusion <p>DO is a relevant and recurring problem in mental health care. In this sense, DO emerges from the interaction between stigma, clinical complexity, and highly demanding environments. The evidence also indicates that some clinical and organizational factors may either increase or reduce risk.</p>