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.