Background <p>Postoperative delirium (POD) is a common and serious complication in older surgical patients. It contributes to multiple adverse outcomes, such as increased mortality, accelerated cognitive decline, and higher healthcare costs. Repetitive transcranial magnetic stimulation (rTMS), a non-invasive neuromodulation technique, has shown potential in improving cognitive performance.</p> Methods <p>In this prospective, single-centre, randomized controlled trial, 122 older patients undergoing lower limb arthroplasty will be enrolled from The First Hospital of Jiaxing, Jiaxing, China. The criteria for the observational indicators are intended to be based on two primary sources: those established in the published literature and those deemed critical based on practical clinical considerations. These indicators are categorized into primary and secondary outcomes. The primary outcome is the POD occurrence within three days post-surgery, as evaluated via the 3-Minute Diagnostic Interview for Confusion Assessment Method-defined Delirium (3D-CAM) or the Confusion Assessment Method for the Intensive Care Unit (CAM-ICU). Secondary outcomes include POD subtypes and severity, postoperative pain intensity, quality of recovery and sleep, inflammatory biomarkers, and opioid dosage expressed as morphine milligram equivalents (MME). All data analysis and visualization are conducted with IBM SPSS Statistics (v25.0) and GraphPad Prism (version 8.0), respectively. Continuous variables are compared using the Student’s t-test or the Mann-Whitney U test. Categorical variables are presented as frequencies (proportions) and analyzed with Pearson’s chi-square test. For non-normally distributed longitudinal data, a generalized estimating equation (GEE) is employed. Mediation analysis will be performed using Model 4 of the PROCESS macro.</p> Discussion <p>This prospective, randomized controlled trial seeks to expand the evidence for using rTMS to prevent POD and to optimize the clinical management of this condition in older patients undergoing lower limb arthroplasty. Specifically, the study aims to determine the efficacy of rTMS in reducing POD incidence, establish a more robust scientific foundation for its clinical use, and explore a new therapeutic approach to improve patient prognosis.</p> Trial registration <p>Chinese Clinical Trial Registry (ChiCTR2500100984).</p>