The sound velocities and single-crystal elastic constants of Fo 8 9 hydrous ringwoodite (γ-Mg 1 . 7 Fe 0 . 2 2 H 0 . 1 6 SiO 4 ) containing ~10,000ppm by weight (1wt.%) H 2 O have been determined from seven separate pure-mode travel-time measurements using gigahertz ultrasonic interferometry. The experiments feature a new Yttrium-Aluminum-Garnet (YAG) P-to-S conversion shear buffer rod (BR), capable of producing pure ultrasonic shear waves with known polarization in the region of 0.5-2.0GHz. To our knowledge they are the first such single-crystal ultrasonic measurements on a high-pressure phase recovered from the multi-anvil press. The cubic single-crystal elastic constants of hydrous Fo 8 9 ringwoodite at ambient conditions are (in GPa): c 1 1 =298+/-13, c 4 4 =112+/-6, and c 1 2 =115+/-6. Hydration of Fo ~ 9 0 ringwoodite to 1wt.% H 2 O reduces the adiabatic bulk (K 0 S =176+/-7GPa) and shear (G=103+/-5GPa) moduli by about 6 and 13%, roughly equivalent to raising the temperature at room pressure by 600 and 1000 o C, respectively. Assuming a linear trend with hydration, we calculate that P- and S-wave velocities are reduced by about 40m/s for every 1000ppm weight (0.1wt.%) H 2 O added to Fo ~ 9 0 ringwoodite. P- and S-wave velocities of the lower transition zone in PREM are consistent with a hydrated ringwoodite-rich composition.