We present observational evidence from coastal tide station and bottom pressure data that basin-mode oscillations are frequently excited in the Japan/East Sea (JES). The fundamental basin-mode is a Kelvin-wave-like oscillation consisting of a single amphidromic system around which the high water propagates counter-clockwise. Its period is about 6.7 hours and its coastal wavelength is equivalent to the circumference of the JES. The relative amplitudes of the observed oscillations agree with Rikiishi's 1986 model results except for stations near the Korea Strait where the closed boundary in the model produces unrealistically high amplitudes. The basin oscillation amplitude varies on synoptic time scales (2--17 days) and exhibits seasonal variations. The optimal wind direction to generate basin-mode oscillations is along 600/2400 T.