This article describes the mineralogy, chemistry, and geotechnical properties of quick and nonquick clay profiles in Ariake Bay, Japan, and the correlations of the mineralogy and chemistry of the clays with their geotechnical properties. The chemistry is discussed in terms of saline environment at the time of deposition, and postdepositional weathering processes. The principal clay minerals identified were smectite, mica, and kaolinite. The salinity in pore water of nonquick clay varied, according to changes in sea level, from brackish to marine, and then brackish toward the surface. The higher iron oxide contents of the clay profiles were attributed to the production of iron hydroxide by pyrite oxidation, due to exposure of the clays to atmosphere for the nonquick clay sediment, and due to the supply of oxygen from fresh water permeating through the clays during leaching process for the quick clay sediment. The major factors which increase the activity of the clay were both smectite and salinity in pore water for the nonquick clay, and smectite for the quick clay. The greater Cu/p values of the clays were associated with the higher iron oxide contents. There was a negative correlation between the sensitivity and salinity in pore water, and a positive correlation between the sensitivity and liquidity index for the combined data on quick and nonquick clays. The compression index of undisturbed clays was found to be represented as a function of the liquid limit and sensitivity.