Mixing at the southwestern boundary of the Ulleung Basin (UB) in the Japan/East Sea was examined using combined ship-based, moored, and quasi-autonomous observations. These observations revealed strong tidal currents, high-frequency internal wave packets, and energetic turbulence over the sill connecting the Korea/Tsushima Strait and the UB, especially when a supercritical flow becomes subcritical over the sill. In addition, strong velocity shear associated with the semidiurnal baroclinic tide was found near the bottom, where the bathymetry slope was critical to the semidiurnal tide. Large temperature fluctuations were observed during the subcritical phase of the flow, as hydraulically controlled flow generated internal-wave packets and energetic turbulence with a turbulent-kinetic-energy dissipation rate of urn:x-wiley:00948276:media:grl64820:grl64820-math-000110−4 Wkg−1, and a diapycnal diffusivity of urn:x-wiley:00948276:media:grl64820:grl64820-math-000210−2 m2s−1. Our analysis suggests that the sill is a hot spot of mixing, which in turn can play an important role in the Japan/East Sea circulation.