The current elasticities of major mantle transition zone minerals and their hydrous counterparts, such as wadsleyite, ringwoodite, and majorite garnet, have the velocity gradients several times lower than those of the Earth models. Here we use ultrasonic interferometry technique to measure the sound velocities of wadsleyite (Mg0.91Fe0.09)2SiO4 at pressures up to 14 GPa and room temperature. Compared with previous studies, we have obtained possibly the highest ∂VP/P and ∂KS/P value at transition zone pressure, which are still noticeably lower than the Earth models, such as the preliminary reference Earth model, ak135, iasp91, etc. The results indicate that the VP contrast of “dry” olivine‐wadsleyite transition at 410 km is higher than previous thought, while that of wadsleyite‐ringwoodite transition at 520 km is noticeably low. Using the current and previous data on “dry” and hydrous wadsleyite a water “gradient” model at upper mantle transition zone were proposed and discussed. Plain Language Summary Olivine and its high pressure polymorphs (wadsleyite and ringwoodite) are the major components in the Earth's upper mantle and mantle transition zone, which dominant the physical properties of the Earth's mantle. However, using the current pyrolitic model and the measured elasticities (velocities and moduli) of wadsleyite and ringwoodite, one cannot reproduce the observed velocity profiles of the Earth models (e.g., preliminary reference Earth model, ak135). Ultrasonic interferometry technique is a mature technique to measure the compressional and shear wave velocities of polycrystalline aggregates. Using this technique, we obtained the sound velocities and their pressure (depth) dependences of iron‐bearing wadsleyite at pressures up to 14 GPa and room temperature. The results suggest that under dry condition, the seismic velocity contrasts at 410 km discontinuity (olivine‐wadsleyite transition) are higher than previous reported. To match the velocity contrasts at 410 km depth and the high velocity gradients at mantle transition zone, a hydrous transition zone with water “gradient” is proposed based on this study and previous results. Key Points Sound velocities of (Mg0.91Fe0.09)2SiO4 Wadsleyite were measured up to 14 GPa at room temperature Possibly the highest ∂VP/P and KS′ values were obtained, which are still much lower than those of the Earth models Water content “gradient” was proposed to match the high velocity gradients in the mantle transition zone