Conventional tuned mass damper (TMD) is a popular and generally accepted vibration control device in the field of passive structural control. However, it was found that the control efficacy of a conventional TMD may significantly degrade when the TMD’s frequency does not tune to its desired value. In addition, the vibration energy of controlled structure transferred into the TMD is dissipated by viscous or friction damper and becomes waste heat. In this paper, a new type of TMD, called electromagnetic TMD inerter (EM-TMDI) is developed by replacing the viscous dampers with electromagnetic rotary transducers so that a more flexible viscous damping can be achieved and part of the energy originally dissipated by the dampers could be harvested. A flywheel with variable mass moment of inertia will be introduced into the transmission system of the TMD to adjust TMD’s frequency to mitigate the frequency detuning effect and to enhance the control efficacy of TMD system. The theoretical derivation is performed to generate the relationship between the DC motor and the transmission system of the EM-TMDI. Optimal design method considering the inerter of rotary transducers will be developed. This study first designed and manufactured a scale-down, double-deck EM-TMDI. A series of shaking table tests were conducted at NCREE Tainan laboratory to verify the capability of inerter to change TMD’s frequency.