Despite widespread interest in variable-speed pump-turbines, the effects of rotational speed on runner modal characteristics remain unclear. This study models the runner as a rotating disk and experimentally investigates its natural frequencies and hydrodynamic damping behaviours over 0–720 r/min. Nodal diameter (ND) modal families and coupled nodal diameter-nodal circle (ND-NC) hybrid modes of the disk were identified, with mode splitting into co-rotating and counter-rotating components observed exclusively in ND families. For all ND modes, the natural frequency of co-rotating modes exhibited a monotonic decrease with increasing rotational speed, whereas counter-rotating modes showed an opposite trend. The resulting frequency gap exhibited a linear dependence on rotational speed, and the center frequency gradually drifted downward with increasing rotational speed. When the reduced frequency is adopted as the independent variable, the natural frequencies of co-rotating and counter-rotating modes were distributed almost symmetrically with respect to the center frequency, for which a second-order polynomial regression model was proposed to characterize this unified behavior. Rotational speed was also found to exert a strong influence on hydrodynamic damping ratios: counter-rotating modes exhibited up to 54.07% lower damping compared to their co-rotating counterparts, highlighting a pronounced asymmetry in fluid-structure interaction. The evolution of damping with rotational speed exhibited two regimes, with nearly constant damping at low speeds and a monotonic increase at higher speeds.