The equilibrium distribution of manganese between slag and molten iron has been investigated under three different gas environments. MgO-saturated CaO–FeO–Al2O3–SiO2–MnO (0.2–0.8 wt%)–P2O5 (0.1–0.9 wt%) slags were studied in the temperature range 1550–1600°C under CO/CO2/H2/H2O (natural/coal gas, NG/CG), H2/H2O (H2 reductant), and CO/CO2 with pO2 = 10−10 to 10−9 atm. The first two gas mixtures represented the case of using natural or coal gas and hydrogen in the novel flash ironmaking technology. The average LMn (manganese distribution ratio) was found to be 4, 5, and 6 under CO/CO2/H2/H2O (natural/coal gas), H2/H2O (H2 reductant), and CO/CO2, respectively. In other words, LMn under H2 and natural/coal gas conditions was 29% and 43% less than under the CO/CO2 atmosphere. Therefore, water in the gas atmosphere depresses LMn. When an ore with Mn content of 0.2 wt% is fed into three routes in addition to coke with 0.07 wt% Mn, which was added at the ratio of 0.4 coke-to-hot metal in the case of the CO/CO2 gas mixture, the hot metal reduced under the CO/CO2, NG/CG, and H2 conditions, is expected to contain 0.13, 0.14, and 0.15 wt% Mn, respectively. That is H2 and NG/CG are expected to produce hot metal with 17% and 25% more Mn than in the CO/CO2 conditions. This work has for the first time determined the effect of water vapor on the distribution of Mn between slag and molten iron.