Speciation of iodine in a soil−water system was investigated to understand the mechanism of iodine mobility in surface environments. Iodine speciation in soil and pore water was determined by K-edge XANES and HPLC−ICP-MS, respectively, for samples collected at a depth of 0−12 cm in the Yoro area, Chiba, Japan. Pore water collected at a 0−6 cm depth contained 50%−60% of organic iodine bound to dissolved organic matter, with the other portion being I⁻. At a 9−12 cm depth, 98% of iodine was in the form of dissolved I⁻. In contrast, XANES analysis revealed that iodine in soil exists as organic iodine at all depths. Iodine mapping of soil grains was obtained using micro-XRF analysis, which also indicated that iodine is bound to organic matter. The activity of laccase, which has the ability to oxidize I⁻ to I₂, was high at the surface of the soil−water layer, suggesting that iodide oxidizing enzymes can promote iodine organification. The distribution coefficient of organic iodine in the soil−water system was more than 10-fold greater than that of iodide. Transformation of inorganic iodine to organic iodine plays an important role in iodine immobilization, especially in a surface soil−water system.