In this paper, an electromechanical actuator is utilized for bidirectional power flow in a tuned mass damper (TMD). The actuator force is controlled to achieve desired mechanical damping and stiffness values for the whole system by controlling the current using a power electronic converter. The resulting TMD can autonomously change its damping and stiffness values to minimize a host structure's displacement under different excitation frequencies. A multi-variable sliding mode extremum-seeking algorithm is used to tune the TMD damping and stiffness levels to optimal values. Analytic and simulation results show that the active TMD exhibits better performance when compared to non-adaptive and robust TMDs.