<p>Ageing is characterised by the coupled decline of somatic and neural integrity. While the global burden of dementia continues to rise, scalable markers that capture this systemic vulnerability to identify individuals at risk remain scarce. Phase angle (PhA), a bioelectrical impedance analysis-derived metric reflecting cellular membrane integrity and body cell mass, has been linked to frailty and mortality; however, its potential as a sentinel for cognitive decline has not been systematically synthesised. We searched five databases up to October 30, 2025, and identified 12 observational studies comprising 7588 adults aged 65 years or older. Methodological quality was independently assessed using the JBI Critical Appraisal Checklist for Analytical Cross-Sectional Studies and the JBI Critical Appraisal Checklist for Cohort Studies by two reviewers. Any inconsistencies were addressed by a third reviewer. Our synthesis reveals a consistent positive association: higher whole-body PhA correlates with better global cognition and lower odds of cognitive impairment, with effect sizes of approximately 0.6 Mini-Mental State Examination points per one-degree increase in PhA. Crucially, segmental analyses identified leg PhA—but not arm or trunk PhA—as a robust correlate of cognitive function, suggesting a specific link via the muscle–brain axis. These findings position PhA, particularly of the lower extremities, as a scalable, non-invasive biomarker integrating somatic and brain ageing. Incorporating PhA into geriatric assessment could refine early risk stratification in primary care, a strategy that now warrants validation in large-scale longitudinal cohorts.</p> Graphical Abstract <p></p>