Giant magnetoimpedance (GMI) in the reentrant spin-glass (SG) phase of ferromagnetic Fe90Zr10 is reported. The temperature (T) dependence of the GMI allows the investigation of the spin dynamics in the SG phase in the MHz frequency regime and thus very short relaxation times τ (∼10−8 s). The GMI shows a broad maximum around 150 K and diminishes with decreasing T below the glass temperature Tg of 15 K. The magnetic permeability data obtained from the GMI data show the general features observed in the ac magnetic susceptibility measured at lower frequencies (10 ≤ f ≤ 104 Hz), yielding values of Tf (=Tg(f)) that allow testing the validity of the power-law scaling used for describing the dynamics of SG-phases up to 15 MHz. A log-log plot of τ (=1/f) versus the reduced critical temperature shows two distinct regimes in the time-domain: (1) a critical slowing-down is observed for values τ > 3 × 10−3 s; and (2) for 7 × 10−8 ≤ τ ≤ 3 × 10−3 s. In the latter case a fitting to the power-law yields the value 7.4 for the product of the critical exponents zν, and 1.6 × 10−7 s for the microscopic relaxation time τ0. The product of the exponents is appropriate for an Ising spin glass.