Lawsonite is a key hydrous mineral in subducting oceanic crust, with an H₂O mass fraction of up to 11.5% and volume fractions that can reach 50%, and 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 1 073 K for 2 h. 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, the adiabatic bulk modulus and shear modulus are 120(1) and 51(1) GPa at room temperature and ambient pressure, and their derivatives with respect to pressure are 5.7(1) and 0.9(1), respectively. 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 about 60–90 km within intermediate-temperature subducting slabs such as Nicaragua.