An innovative Semi-Active Pendulum Tuned Mass Damper (SAPTMD) with variable frequency is introduced in this paper. The SAPTMD consists of two identical pendulum systems, each consisting of a pendulum and two springs attached to the pendulum mass. The pendulums are mounted on two boxes that can rotate symmetrically around the horizontal shafts that pass through them. Due to the change in inclination of the pendulum systems and, consequently, the orientation of the oscillating pendulums, the frequency of the proposed system can change within a predefined range. Besides, the need for less height for installation is another advantage of the SAPTMD over the conventional pendulum tuned mass dampers. In order to show the efficiency of the SAPTMD, it is attached to a single degree of freedom primary system excited similar to excitation due to an unbalanced rotating machinery during its start-up period. The SAPTMD’s performance is compared with a classical tuned mass damper. As it is shown, the response of the primary system is reduced noticeably. Furthermore, the performance of the system is improved by utilizing a fuzzy Proportional-Integral-Derivative (PID) controller.