This study examines the effect of gas hydrate formation on seismic wave velocities of fine-grained sediments. Synthesis of gas hydrates in fine-grained sediments has proved to be challenging, and how hydrate formation would affect the seismic wave velocities and stiffness of clay-rich sediments has not yet been fully understood. In this study, CO2 hydrate was synthesized in remolded and partially water-saturated clayey silt sediments that were originally cored from a hydrate occurrence region in the Ulleung Basin, East Sea, offshore Korea. After achieving excess water conditions, compressional wave and shear wave velocities were measured for different hydrate saturations and under different vertical effective stresses. The results reveal that the compressional wave velocity VP and shear wave velocity VS increase, and the stress-dependency of VP and VS decreases as the hydrate saturation SH increases from 0% to ~60%. In particular, the VS-SH trend lies between the grain-cementing model and the load-bearing model, suggesting that gas hydrate formation in clayey silt sediments causes weak cementation from a hydrate saturation less than ~28%. The weak cementation in fine-grained sediments can be explained by the breakage of hydrate bonds that are cementing grains during sediment compression and/or the innate weakness in bonding between hydrate crystals and fine mineral grains owing to the presence of unfrozen water films on clay mineral surfaces. In addition, it is found that at low SH, the cementation effect on VP is masked by the high stiffness of pore-filling phases, but it becomes pronounced at SH greater than 47%.