High-pressure elasticity studies will play a central role in efforts to constrain the potential hydration state of the Earth’s mantle from seismic observations. Here we report the effects of 1 wt% H2O (as structurally bound OH) on the sound velocities and elastic moduli of single-crystal ringwoodite of Fo90 composition, thought to be the dominant phase in the deeper part of the transition zone between 520 and 660-km depth. The experiments were made possible through development of a GHz-ultrasonic interferometer used to monitor P and S-wave travel times through micro-samples (30-50 µm thickness) under hydrostatic compression in the diamond- anvil cell. The velocity data to ~10 GPa indicate that hydrous ringwoodite supports 1-2% lower shear-wave velocities than anhydrous ringwoodite at transition zone pressures, though elevated pressure derivatives (K’ = 5.3 ± 0.4 and G’ = 2.0 ± 0.2) bring calculated hydrous P-velocities close to anhydrous values within their mutual uncertainties above ~12 GPa. Corresponding VP/VS ratios are elevated by ~2.3% and not strongly dependent on pressure. Velocities for hydrous ringwoodite are calculated along a 1673 K adiabat using finite-strain theory and compared with existing data on anhydrous ringwoodite and various radial seismic models. It may be possible to distinguish hydration from temperature anomalies by low S-velocities associated with "normal" P-velocities and accompanying high VP/VS ratios. The presence of a broadened and elevated 410-km discontinuity, together with depressed 660-km discontinuities and intervening low S-wave anomalies along with high VP/ VS ratios are the most seismologically diagnostic features of hydration considering the available information from mineral physics.