The demand for large mass limits the engineering application of traditional nonlinear energy sinks. In this paper, a novel type of nonlinear energy sink is proposed to overcome this drawback. The mass in the traditional nonlinear energy sink is replaced by an inerter. A substantial reduction in the mass of nonlinear energy sinks has been achieved. At the same time, the effect of vibration suppression is improved. Introducing the inertance without the mass, the equation of motion for the inertial nonlinear energy sink is established. By applying the harmonic balance method, the nonlinear forced vibration response of the system is predicted. The analytical results are supported by numerical simulations. By comparing with the traditional nonlinear energy sink, the benefits of using inertial nonlinear energy sink are explored. For examples, the additional mass required to achieve the same vibration suppression effect is smaller. The same additional mass enables better vibration suppression. Moreover, the optimum mass ratio is searched by the maximum amplitude trajectory. Furthermore, amplitude-frequency responses are generated for different parameters. The numerical results demonstrate that the inertial design could enhance the vibration suppression performance. In general, this work shows that the inertial design can significantly reduce the additional mass. Therefore, this study promotes the practical application of the NES.