This report documents clay mineral assemblages outboard of the Nankai Trough at two sites in the Shikoku Basin. The sites make up part of the Kumano transect, offshore south-central Japan. Coring began during Integrated Ocean Drilling Program (IODP) Expedition 322, as part of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE). IODP Site C0011 is located on the northwest flank of Kashinosaki Knoll, and IODP Site C0012 is located on the summit of the seamount. A total of 292 samples of hemipelagic mud and mudstone were analyzed by X-ray diffraction, using oriented aggregates of the clay-size fraction (<2 mu m). Smectite varies the most among the clay-size constituents, ranging in abundance from 35% to 100% (where smectite + illite + chlorite + kaolinite + quartz = 100%). Specimens with smectite contents of 80% or more are classified as bentonites, and we identified 44 such samples. The estimated amount of smectite in bulk mudstone ranges from 24% to 87% (where total clay minerals + quartz + feldspar + calcite = 100%), and a substantial number of bulk samples contain >45% smectite. On average, the expandability of illite/smectite mixed-layer clay is equal to 77%, and the average proportion of illite in illite/smectite is 11%. There is no systematic progression of clay mineral diagenesis over the depths sampled. Most values of illite crystallinity index are consistent with a detrital source that contains sedimentary rocks that were exposed to anchizone conditions of incipient metamorphism. The smectite-rich clay mineral assemblage is typical of early to middle Miocene deposits within the lower part of the Shikoku Basin.