The Fukushima nuclear accident in March 2011 has released a large amount of radioactive pollutants to the environment. Of the pollutants, iodine-129 is a long-lived radionuclide and will remain in the environment for millions of years. This work first report levels and inorganic speciation of 129I in seawater depth profiles collected offshore Fukushima in June 2011. Significantly elevated 129I concentrations in surface water were observed with the highest 129I/127I atomic ratio of 2.2 × 10−9 in the surface seawater 40 km offshore Fukushima. Iodide was found as the dominant species of 129I, while stable 127I was mainly in iodate form, reflecting the fact that the major source of 129I is the direct liquid discharges from the Fukushima NPP. The amount of 129I directly discharged from the Fukushima Dai-ichi nuclear power plant to the sea was estimated to be 2.35 GBq, and about 1.09 GBq of 129I released to the atmosphere from the accident was deposited in the sea offshore Fukushima. A total release of 8.06 GBq (or 1.2 kg) of 129I from the Fukushima accident was estimated. These Fukushimaderived 129I data provide necessary information for the investigation of water circulation and geochemical cycle of iodine in the northwestern Pacific Ocean in the future.