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The horizontal structure of deep mean circulation and its seasonal variability in the Japan/East Sea (JES) were studied using profiling float and moored current meter data. The deep circulation in the Japan Basin (JB) flows cyclonically, basically following f/ H contours. The correlation between the directions of deep current and f/ H contour increases as |▿( f/ H)| increases, reaching remarkably high correlation coefficient (>0.8) values in steep slope regions in the JB. In contrast to the JB, the deep mean circulation in the Ulleung/Tsushima Basin (UTB) is generally weak and cyclonic accompanied by sub-basin-scale cyclonic and anticyclonic eddies. The UTB shows a poorer correlation between directions of deep current and f/ H contours than other basins. The time-space averaged deep mean current is about 2.8 cm/s and the volume transport in the deep layer (800 m to bottom) in the JB reaches about 10 Sv (10 × 106 m3s−1), which is about four times greater than the inflow transport through the Tsushima Straits. A salient feature is that the amplitude of deep mean current in the JB reveals a remarkable seasonal variation with a maximum in March and minimum in October. The annual range of the seasonal variation is about 30% of the mean velocity, whereas that in the southern JES (UTB and Yamato Basin) is weak. [ABSTRACT FROM AUTHOR]