A tunable mass damper (TMD) is provided that can be coupled to a rotating inertia apparatus such as a reaction wheel assembly (RWA). The TMD includes a housing that contains a flexure, a mass, and damping fluid within the housing. The housing is coupled to the flexure, and the flexure is coupled to the mass. The mass is free to sway in the damping fluid. The damping fluid envelops the mass and provides damping between the mass and the housing. At least one of the mass, flexure and the damping fluid can be adjusted to tune the TMD to produce a resonance at or near a mode that is to be mitigated by the TMD such that it operates at a desired or optimal operating frequency.