The oxidation behavior of iodide ion (I−) was investigated in aqueous solutions under a high dose rate of gamma irradiation in the range of 0 to 10 kGy·h−1. In particular, we investigated the formation of tri-iodide ion (I3−), the pH change of the solution, and the behavior of iodine species after the irradiation. As the gamma dose and the irradiation time increased, both the formation rate and the amount of I3− correspondingly increased. While I3− is not present above pH 10 due to its disproportionation reaction even without gamma irradiation, with irradiation, I3− does not exist above pH 6.4 because the H2O2 acts as a reductant above pH 5.4. At relatively high concentrations of I−, I3− was the major oxidation species of the gamma irradiation. However, as the irradiation progressed, the concentration of I3− decreased gradually, and eventually, I2 was left as the only species.