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δ-(Al,Fe)OOH is considered to be one of the most important hydrous phases on Earth, remaining stable under the extreme conditions throughout the mantle. The behavior of δ-(Al,Fe)OOH at high pressure is essential to un-derstanding the deep water cycle.δ-(Al0.956Fe0.044)OOH crystals synthesized at 21 GPa and 1473 K were inves-tigated by high-pressure Brillouin light scattering spectroscopy and synchrotron X-ray diffraction up to 135.4 GPa in diamond anvil cells. The incorporation of 5 mol％FeOOH increases the unit-cell volume ofδ-AlOOH by～1％and decreases the shear-wave velocity (VS) by ～5％ at 20–135 GPa. In particular, the compressional (VP) and shear (VS) wave velocities ofδ-(Al0.956Fe0.044)OOH are 7％–16％and 10％–24％greater than all the major min-erals in the mantle transition zone including wadsleyite, ringwoodite, and majorite. The distinctly high sound velocities ofδ-(Al0.956Fe0.044)OOH at 20–25 GPa may contribute to the seismic anomalies observed at～560–680 km depths in the cold and stagnant slab beneath Izu-Bonin and/or Korea. Furthermore, the VS ofδ-(Al0.956Fe0.044) OOH is about 10％ and 4％–12％ lower than iron-bearing bridgmanite Mg0.96Fe0.05Si0.99O3 and ferropericlase (Mg0.92Fe0.08)O, respectively, under the lowermost mantle conditions, which might partially contribute to the large low-shear-velocity provinces and ultralow velocity zones at the bottom of the lower mantle.