Brucite, a key constituent mineral in hydrated peridotites within subduction zones, can occupy up to the volume fraction of 15% of these water-saturated rocks. Investigating the high-pressure elastic wave velocities of brucite is thus crucial for understanding the composition, seismic velocity structure, and deep-water cycling processes of hydrated peridotites in subduction zones. In this study, dense polycrystalline brucite was synthesized from Mg(OH)₂ reagent under 4 GPa and 523 K for 2 h. The elastic wave velocities and moduli of brucite were measured up to 14 GPa using ultrasonic interferometry. The results demonstrate that the elastic wave velocities and moduli of brucite increase with increasing pressure. By integrating seismic tomography with mineral assemblage modeling, we constrained the water content in the low-velocity anomaly regions of the mantle wedge using the Voigt-Reuss-Hill (VRH) model. Our estimations indicate that the water mass fraction ranges from 3.0%–10.0% in low-velocity anomaly zones of the mantle wedge above the subducting slab at depths of 20–40 km, and 1.0%–3.0% within the subducting slab at depths of 60–80 km beneath northeastern Japan.