A method was developed for the determination of 129I in soil samples using a Triple Quadrupole ICP-MS, with the objective of investigating radioiodine released by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. The determination of 129I by ICP-MS is capable of providing a high sample throughput compared to other methods. Nonetheless, the high background caused by 129Xe impurities in argon plasma gas and polyatomic ions such as 127IH2+ and 127ID+ has made it difficult to carry out determinations of this isotope by conventional ICP-MS instruments. In this study, oxygen was used as a reaction gas for reducing the background intensity of m/z 129, principally by 129Xe. The contribution of polyatomic ions such as 127IH2+ and 127ID+ could be effectively corrected for by assuming a production ratio of 5 × 10−9. The detection limit for 129I in solution was successfully improved. The measured 129I/127I ratios in NIST SRM 3231 Level II standard solution are consistent with the certified value of 0.981 × 10−8 within the analytical error, suggesting the reliability of the method to the measurement of 129I/127I. In order to confirm the applicability of this method to the measurement of 129I/127I ratios of Fukushima soils, samples were collected for analysis from 5–60 km away from the FDNPP. Prior to the ICP-MS measurement iodine fractions were separated from soil by pyrohydrolysis and were purified by solvent extraction. The 129I/127I ratios in the samples are consistent with values determined by accelerator mass spectrometry (AMS) within the analytical error. This method provides a powerful tool for the investigation of radioiodine contamination in Fukushima and elsewhere.