New simultaneous seismic reflection and physical oceanographic observations east of Japan demonstrate the utility of the seismic reflection method in mapping oceanic fine structure. Synthetic seismograms calculated from temperature and salinity data confirm that seismic reflections correlate with physical oceanographic structures. Seismic reflections at the boundary between the warm Kuroshio and the cold Oyashio water masses correspond to well developed, ~10 m scale, temperature fine structure. Vertical current profiles suggest that this fine structure is caused by interleaving of these two water masses. We compare our seismic images with acoustic Doppler current profiler (ADCP) intensity maps and discuss similarities and the differences between seismic images and the ADCP maps. Our study demonstrates that even relatively low-energy seismic sources, in this case a 3.4 l (210 in3) generator-injector (GI) airgun, can be used to image upper oceanic fine structure.