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Elastic properties of Si0.95H0.21O2 with hydrogarnet substitution (4H+ = Si4+) across the post‐stishovite transition (28–42 GPa) are determined up to 70 GPa using ab initio calculations and a pseudo‐proper type Landau model. At 28 GPa, elastic coefficients C11 and C12 converge, and the average shear and compressional velocity (VS and VP) decrease by a maximum of 25.5% and 5.2%, respectively. Hydrogarnet substitution reduces ambient elastic moduli and sound velocities, and shifts shear softening to lower pressure. 2%–13% Si0.95H0.21O2 may cause a VS anomaly of −0.5% to −2.6% at 700–820 km depth, explaining low VS layers beneath North America and the European Alps. Additionally, 20 vol % SiO2 in subducted basalt, with decreasing water content from 3.2 wt% to zero, could cause a VS anomaly of up to −7(4) % from 700 to 1,900 km depth, aligning with seismic scatterers identified in some subduction regions. Plain Language Summary: Our study explores the elasticity of stishovite and post‐stishovite (high‐pressure forms of SiO2) with 3.2 wt% water (Si0.95H0.21O2) up to 70 GPa using theoretical methods. During the transition from stishovite to post‐stishovite (28–42 GPa), shear softening occurs, and shear wave speed (VS) decreases by up to 25.5%. The direct hydrogen substitution for silicon can lower the ambient seismic wave speed and shift shear softening to lower pressure. A small amount of Si0.95H0.21O2, when present in the top lower mantle, could display low VS under North America and the European Alps, as identified by seismologists. Also, subducted mid‐ocean ridge basalt with lower H2O content in SiO2 from 700 to 1,900 km depth might account for seismically observed negative VS anomalies in certain subduction regions. Key Points: Elasticity of Si0.95H0.21O2 with 4H+ = Si4+ across the post‐stishovite transition is studied by ab initio calculation and Landau theoryElastic coefficient C11 converges with C12 and shear wave velocity VS decreases by 25.5% at 28 GPa in Si0.95H0.21O2Low VS layers and small‐scale VS anomalies in the shallow lower mantle could be caused by the post‐stishovite transition in hydrous SiO2 [ABSTRACT FROM AUTHOR]