Objective <p>To evaluate the clinical relevance of CT–derived deltoid muscle attenuation as an imaging-based indicator of muscle quality and its association with clinical outcomes in geriatric patients with proximal humerus fractures.</p> Materials and methods <p>In this retrospective multicenter cohort study, 118 patients aged 65&#xa0;years and older with three- or four-part proximal humerus fractures were included. Patients were treated either non-operatively or with reverse shoulder arthroplasty. Deltoid muscle attenuation and cross-sectional area were measured on preoperative CT images at a standardized axial level. Patients were stratified according to a median attenuation threshold. Clinical outcomes at one year were assessed using the Constant-Murley score, Disabilities of the Arm, Shoulder and Hand score, Visual Analog Scale, and subjective shoulder value.</p> Results <p>No significant correlation was observed between deltoid muscle cross-sectional area and attenuation. Patients with higher attenuation demonstrated significantly better functional outcomes, including higher Constant-Murley and subjective shoulder value scores and lower Disabilities of the Arm, Shoulder and Hand and Visual Analog Scale scores. In this subgroup, reverse shoulder arthroplasty was associated with superior outcomes compared to non-operative treatment. In contrast, among patients with lower attenuation, no significant differences were observed in functional scores between treatment groups, although lower pain levels and higher subjective satisfaction were noted following surgical treatment.</p> Conclusion <p>Preoperative deltoid muscle attenuation derived from CT is associated with clinical outcomes in geriatric proximal humerus fractures and may serve as a practical imaging-based parameter for patient stratification. These findings support the potential role of muscle quality assessment in preoperative evaluation and treatment decision-making.</p>