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Seismic anomalies in subducted slabs, including low velocity zones and shear wave splitting, have often been related to hydrous regions. Phase D (MgSi₂H₂O₆, 10–18 wt.% H₂O) may be the ultimate water carrier in hydrous subducted peridotite and its seismic properties are thus essential for interpreting observed anomalies in terms of hydration. Here, we report the sound velocities and elasticity of Mg‐ and Al‐Fe‐bearing phase D single‐crystals measured by Brillouin spectroscopy. The room conditions adiabatic bulk and shear modulus are: K_S0 = 154.8(3.2) GPa and μ = 104.3(2.1) GPa for Mg‐phase D and K_S0 = 158.4(3.9) GPa and μ = 104.7(2.7) GPa for AlFe‐phase D, suggesting minor effect of cation substitution on the elasticity of phase D. Based on the seismic velocity data, we found that 16 vol.% of AlFe‐phase D in hydrous subducted peridotite with 1.2 wt.% H₂O could provide a plausible explanation for …