A novel tunable mass damper (TMD) is developed using the sensitivity of transversal bending stiffness and resonance frequencies of a beam to its axial force. This smart TMD consists of a force actuator-sensor unit suspended in a rigid frame by two flexible beams coupled to the axial ends of the unit and the frame. The force actuator-sensor unit is composed of a giant magnetostrictive composite-based force actuator for producing an axial force to the beams and a pair of piezoelectric ceramic-based force sensors for generating a tuning signal. Through adjusting the magnetic field strength applied to the force actuator to change the axial force exerted on the beams, the transversal bending stiffness of the beams and hence the natural frequency of the smart TMD is tuned. In this paper, the design, fabrication, and characterized of the smart TMD is described. The measured resonance frequency of the smart TMD is 65 Hz at zero magnetic tuning field and 50 Hz at an applied magnetic field of 686 Oe. Tunability of the resonance frequency as high as 23 % is achieved with the reasonably low magnetic tuning field. The frequency response functions as measured using the force sensors agree well with those obtained using a commercial accelerometer, indicating a great possibility of directly deploying the force sensors for active or semi-active tuning or control purposes.