AbstractCompressional and shear wave velocities of topaz have been measured by ultrasonic interferometry in a multi‐anvil apparatus up to 13 GPa at room temperature. By fitting the measured velocities under pressure to finite strain equations, the elastic moduli of topaz and their pressure derivatives were obtained, yielding K_S0 = 165.5 (25) GPa, K_S′ = 2.9 (1), G₀ = 116.5 (17) GPa, and G′ = 1.0 (1). Modeling of the velocity profiles of subducted sediments suggests that at 8 GPa (∼240 km depth) the P‐ and S‐ wave velocities of the subducted sediments exhibit a first order increase of 10% and 11%, respectively. The comparison between seismic observations and velocity contrasts of sediment + MORB model at 240 km shows dehydration of a 1.5–3.9 km thick sediment layer can potentially contribute to the seismically observed Lehmann discontinuity in Tonga, Japan, Philippine Sea, South America, Indonesia, and South Shetland subduction zones.