In this work we report tritium concentrations determined in seawater samples collected offshore of Fukushima in the northwestern Pacific Ocean, immediately after the Fukushima Dai-ichi Nuclear Power Plant (F1-NPP) accident. We found surface seawater to have high concentrations of tritium (³H) and cesium-137 (¹³⁷Cs) with respect to the concentrations expected for the investigated region. Tritium concentrations were up to six times the pre-accident level. However, these concentrations were found to be relatively low compared to those of ¹³⁷Cs and ¹²⁹I. This is most likely because of the very low production ratio of tritium to ¹³⁷Cs in the F1-NPP, and the inherently high background concentration of tritium in these environmental waters (mainly ascribed to global fallout from nuclear weapons tests conducted during the 1960s). The tritium distribution in surface seawater showed higher concentrations close to the shore and lower concentrations offshore. Higher tritium concentration areas had spread both northward and southward from the F1-NPP along the coast, indicating that large amounts of tritium were carried by coastal currents. A positive correlation was found to exist between ³H and ¹³⁷Cs concentrations in the seawater. The calculated ³H and ¹³⁷Cs ratio was similar to the production ratio of these isotopes reported for the broken reactors in the F1-NPP, which indicates that both radionuclides might have originated from the F1-NPP. Direct emission of tritium to the ocean from the F1-NPP was estimated to be approximately 0.05 PBq immediately after the accident, which is much smaller than the total inventory in the environment.