A novel seismic control system, referred to as Ridge-Isolated Tuned Mass Damper (RITMD) is introduced; combined of the features of a Tuned Mass Damper (TMD) and seismic isolation. In RITMD, the building’s ridge acts as a secondary oscillator, and the installation of seismic isolators at the ridge level allows for large displacements. Instead of transferring the entirety of seismic energy to the main structure, a significant portion is diverted to the ridge, which effectively reduces the building’s seismic vibrations. To evaluate the performance of the RITMD system, a five-story steel moment-resisting frame was subjected to nonlinear dynamic analysis under two far-field (El Centro and Hachinohe) and two near-field (Kobe and Northridge) earthquakes. Initial modeling was conducted in ETABS, and nonlinear analyses were carried out using SeismoStruct. Results indicated that implementing the RITMD system significantly reduced seismic responses, including story displacements, accelerations, inter-story drifts, and base shear. Specifically, in the El Centro earthquake, story displacements were reduced by up to 36.71%, accelerations by 38.55%, and drifts by 36.71%. In the Kobe earthquake, these reductions were 14.90%, 17.30%, and 17.51%, respectively. On average, the system achieved reductions of 20% in displacements, 15% in accelerations, 18% in drifts, and 20% in base shear. With its applicability to both new and existing structures, low cost, quick implementation, and effective performance, the RITMD system represents a practical solution for enhancing seismic safety in buildings.