Active variable stiffness (AVS) systems have been demonstrated to be effective in response control of buildings subjected to earthquake excitations. Based on the theory of variable structure system (VSS) or sliding mode control (SMC), control methods for applications of AVS systems to seismic-excited buildings are presented. In addition to full-state feedback controllers, general static output feedback controllers as well as special output feedback controllers using only collocated sensors are also presented. Simulation results demonstrate that the performance of the control methods presented is remarkable, and that the performance of output feedback controllers is comparable to that of the full-state feedback controllers.