Abstract This paper presents a comprehensive laboratory experimental characterization of the host sediment from natural hydrate-bearing sediments recovered during the Ulleung Basin Gas Hydrate Expedition 2 (UBGH2) in 2010. The basic geotechnical properties are characterized and compared with previously reported data. The mineralogy and microstructure analyses confirm that the sediments mainly consist of illite, kaolinite, chlorite, calcite, and montmorillonite, with a high concentration of microfossils (diatoms). The presence of microfossils may affect many of the geotechnical properties, including plasticity, activity, specific surface, and grain size. The tested specimens have a relatively high initial void ratio, with a high compression index (e.g., Cc ranging from 0.6 to 1.0) and swelling index (e.g., Cs ranging from 0.1 to 0.25). The shear wave velocities measured during loading indicate a high sensitivity to the stress state, which demonstrates the critical role of effective stress in controlling the stiffness of the sediments. The peak friction angles are determined to be 17.6°–24.6° for clay and silt sediments, and 27.1° for high sand fraction specimens. The stress dependent and strength parameters obtained in this study may be very useful for performing stability analyses during hydrate dissociation and production.