This study explores the effectiveness of low-mass Tuned Mass Dampers (TMDs) for the seismic retrofit of existing reinforced concrete (RC) arch bridges. A case study bridge, located in the Valvestino Valley in Northern Italy and designed in the late 1950s by the renowned engineer Riccardo Morandi, is presented. The structure comprises a 90-meter-span box-section arch supporting a 127.7-meter-long deck via spandrel columns. Nonlinear time-history (NLTH) analyses of the existing configuration revealed critical vulnerabilities under Ultimate Limit State (ULS) seismic loading, primarily due to reinforcement corrosion, insufficient shear detailing, and limited ductility. In response, a retrofit strategy was proposed, combining unidirectional bearings and hysteretic dampers in the longitudinal direction, and low-mass TMDs (μ ≈ 1.9%) to mitigate transverse motions. A parametric study on the mechanical properties of the TMDs was carried out, revealing that an optimized configuration led to a 41.2% reduction in transverse drift and significantly reduced stresses in the most fragile columns. The robustness of the solution was confirmed under varying corrosion scenarios and multi-directional seismic inputs. The results confirm the effectiveness of low-mass TMDs in enhancing seismic performance without altering the structural identity of existing RC arch bridges.