Ultrasonic P- and S-wave velocities of a garnetite sample synthesized from MORB glass at around 17 GPa and 1200°C were measured up to 14.3 GPa at room temperature. The garnetite consists of well-sintered microcrystalline majorite garnet and a minor amount of stishovite. P- and S-wave velocities of the garnetite at room temperature are significantly lower than those of (Mg0.9, Fe0.1)2SiO4 wadsleyite and ringwoodite, which are the major high-pressure phases in the mantle transition region. Bulk and shear moduli of the garnetite are determined from the P- and S-wave velocities, and those of majorite garnet in the MORB composition (MORB majorite) are calculated using the modal abundance data and elastic properties of stishovite. The MORB majorite is estimated to have quite different bulk and shear moduli (up to 19% and 9%, respectively) from those of Mg end-member garnets with simpler chemical compositions.