Anthropogenic radionuclides were released into the environment in large quantities by the Fukushima Daiichi Nuclear Power Plant (1FNPP) accident. To evaluate accident-derived <sup>129</sup>I, the <sup>129</sup>I concentrations in seawater before and after the accident were compared. <br><br> Before the accident (2008–2009), the <sup>129</sup>I concentrations in the western margin of the North Pacific between 32° N and 44° N showed a latitudinal gradient that was expressed as a linear function of latitude. The highest and average <sup>129</sup>I concentrations after the accident were 73 times and approximately 8 times, respectively, higher than those before the accident in this study area. Considering the distribution of <sup>129</sup>I in surface seawater, the accident-derived <sup>129</sup>I in the southern and northern stations of the 1FNPP was predominantly supplied by seawater advection and atmospheric deposition (including microbial volatilization), respectively. <br><br> As of October 2011, depth profiles of <sup>129</sup>I revealed that <sup>129</sup>I originating from the 1FNPP existed mainly in the upper 100 m depth. From the depth profiles, the cumulative inventories of accident-derived <sup>129</sup>I were estimated to be (1.6–9.6) × 10<sup>12</sup> atoms m<sup>−2</sup> in this study area. <br><br> On the basis of the <sup>129</sup>I data in the seawater near Fukushima, the effective dose of <sup>129</sup>I from seafood ingestion was much smaller than the annual dose limit