This article proposes an idea for the development of a novel frequency-tunable electromagnetic energy harvester, which mainly consists of a frequency-tuning system component and an electromagnetic energy-harvesting component. A magneto-rheological elastomer, a kind of smart material, was utilized in the design of the frequency-tuning part using its unique rheological characteristic that its shear modulus can be altered by changing the strength of an external magnetic field. When external excitation is provided to the system, the tip magnet oscillates relative to the coil to produce electricity. The stiffness of the system's equivalent torsional spring composed of magneto-rheological elastomer blocks can be altered by changing the gap distance between two tuning magnets, which results in a shift in the primary natural frequency of the system and significant improvement of energy-harvesting efficiency. This article presents the detailed process for the design, simulation, experiment, and fabrication of the proposed energy harvester. Experiments and numerical simulations were also conducted under band-limited random excitation to support the validity of the present system.