This paper proposes application of single and multiple semiactive variable stiffness tuned mass dampers STMD/SMTMD for response control of multistory structures under several types of excitation. A new semiactive control algorithm is developed based on real-time frequency tracking of excitation signal by short time Fourier transform. A parametric study is performed in the frequency domain to investigate the dynamic characteristics and effectiveness of STMDs. Time history responses of single-degree-of-freedom and five-degree-of-freedom structures equipped with STMDs at the roof level, subjected to harmonic, stationary, and nonstationary excitations are presented. STMD/SMTMD are most effective when they have low damping ratios and the excitation frequency can be tracked. They are superior than their passive counterparts in reducing the response of the main structure both under force and base excitations. In case the fundamental frequency changes due to damage or deterioration of the main structure then the TMD will be off-tune; hence, it will lose its effectiveness significantly, whereas, the STMD is robust against such changes as it is always tuned to the excitation frequency.