To understand the effect of hydration on the elastic properties of silicate melts, we conducted in situ high-pressure Brillouin scattering measurements on two hydrous basalt glasses with different water contents in diamond anvil cells. Second-order phase transitions were observed in the hydrous basalt glasses and are due to the topological rearrangement of the silicate network to a high [Si, Al]–O coordination. Up to a pressure of 10 GPa at 300 K, the extra 2.23 wt% H₂O lowers the elastic moduli of FX-2 basalt glass (2.69 wt% H₂O) by 10%–18%, but does not affect the pressure derivatives of the elastic moduli, compared with FX-1 (0.46 wt% H₂O) basalt glass. The phase transition takes place at a higher pressure in FX-2 compared with FX-1, possibly because of the depolymerization of water to silicate glass. Water interacts with network-forming cations and creates Si–OH and Al–OH groups, and prohibits nonbridging oxygen ions from being connected to other nearby framework cations (i.e., ^[5,6](Si, Al)), resulting in the hysteresis of the second-order phase transition. The density contrasts of our hydrous basalt melts with previous mid-ocean ridge basalt and preliminary reference Earth model data indicate that basalt melts may need very low water content (<0.46 wt% H₂O) to maintain gravitational stability at the base of the upper mantle. Our results show that the elastic properties of hydrous silicate melts may have important implications for the dynamic evolution and chemical differentiation of the mantle.