Temperature, salinity, chlorophyll a, and surface CO2 fugacity (fCO2) were extensively investigated in the Ulleung Basin of the East/Japan Sea during four seasonal cruises. In spring, surface fCO2 showed large variations ranging from 260 to 356 μatm, which were considerably lower than the atmospheric CO2 levels. Surface fCO2 was highest (316 to 409 µatm) in summer. The central part of the study area was undersaturated with respect to atmospheric CO2, while the coastal and easternmost regions were oversaturated. In autumn, the entire study area was fairly undersaturated with respect to atmospheric CO2. In winter, surface fCO2 ranged from 303 to 371 µatm, similar to that in autumn, despite the much lower sea surface temperature. The seasonal variation in surface fCO2 could not be explained solely by seasonal changes in sea surface temperature and salinity. The vertical mixing, lateral transport, and sea-air CO2 exchange considerably influenced the seasonal variation in surface fCO2. The Ulleung Basin of the East/Japan Sea was a sink of atmospheric CO2 in spring, autumn, and winter, but a weak source of CO2 to the atmosphere in summer. The annual integrated sea-air CO2 flux in the Ulleung Basin of the East/Japan Sea was -2.47 ± 1.26 mol m-2 yr-1, quite similar to a previous estimate (-2.2 mol m-2 yr-1) in the south East/Japan Sea. This indicates that the Ulleung Basin of the East/Japan Sea acts as a strong sink of atmospheric CO2.