Abstract: Lawsonite is a key hydrous mineral in subducting oceanic crust, with an H₂O content of up to 11.5 wt.% and volume fractions that can reach ~50 vol.%, and it is stable over a wide pressure–temperature range. Constraining the high-pressure elastic properties of lawsonite is therefore essential for understanding seismic velocity structures in subduction zones and the deep water cycle. In this study, dense polycrystalline lawsonite was synthesized by hot-pressing natural lawsonite powder at 7 GPa and 1073 K for 2 hours. Ultrasonic interferometry was then used to measure P- and S-wave velocities of lawsonite up to 8 GPa at room temperature, from which the elastic moduli and their pressure derivatives were determined. The results show that both elastic wave velocities and moduli increase with pressure, yielding K_S0=120(1)GPa, G₀=51(1)GPa, K_s^'=5.7(1), G^'=0.9(1). Using these elastic properties, together with previous mineral-physics results, we constructed a lawsonite-bearing eclogite model, which indicates that the presence of lawsonite can provide a plausible explanation for the seismic velocity anomalies observed at depths of ~60–90 km within intermediate-temperature subducting slabs such as Nicaragua.