In this paper, we present a flywheel that can adaptively generate variable equivalent mass in response to application requirements. The motivation for the design comes from the need to achieve passive inertial mass, which eventually will lead to passive vibration isolation. This flywheel features a “host” flywheel frame with four sliders, each in a separate track. As the rotational speed of the variable inertia flywheel changes, the distance between sliders and rotation center changes, leading to a variable equivalent mass. The mathematical model of the flywheel is developed to examine its performance. The flywheel is mounted on a two-terminal hydraulic device to test its behavior. Experimental work has also been carried out to identify the parameters of the system (hydraulic device plus flywheel). The mathematical model with the identified parameters is then validated experimentally. During the experiments, the variable inertial force generated by the variable inertia flywheel in response to the changes in the excitation inputs is in good agreement with the prediction of the mathematical model, with the exception of spikes due to backlash of the two-terminal hydraulic system. The proposed design and experimental approach could inspire other passive variable inertial mass control of vibration systems.