Water activity (Aw) is one of the physicochemical properties that may influence microbial activity in deep subseafloor environments; however, Aw for subseafloor sediments has never been examined, even at shallow depths. This study investigated Aw data obtained from core samples collected during Integrated Ocean Drilling Program Expedition 337 in the deep-water coal-bearing basin off Shimokita, Japan. Aw did not show any depth dependence and was relatively low in coal-bearing layers. Aw at depths of 0–2466 meters below seafloor ranged from 0.95 to 0.98, which is quite high and well suited to sustaining microorganisms. Aw for sedimentary rocks was less affected by lithology and porosity than it was by the NaCl concentration and degree of fluid saturation. In addition, the Aw measurements performed in this study yielded results that corresponded closely with values estimated using Raoult’s law and interstitial water chemistry. It therefore appears that Aw for deep-marine sediments is strongly affected by pore water chemistry. The low Aw anomaly in the coalbed unit is considered to be due to contamination by drilling mud and fluid into core samples.