Red mud (RM) is an alkaline industrial solid waste discharged from Bayer process, and its high alkalinity is considered as a serious influence on environment and application. Dealkalization is prerequisite and basis for the complete utilization of RM. Mn-containing leachate is a weakly acidic wastewater generated during the storage of manganese sulfate residue. In the current study, Mn-containing leachate was used to investigate the multistage dealkalization behavior of RM. The results showed that Mn-containing leachate effectively lowered pH levels from 11.82 to 7.61 and the leaching efficiency of Na and K reached 80% and 73%, respectively, under the conditions of five-stage leaching with reaction temperature of 95 °C, reaction time of 2.0 h, and a liquid-solid ratio of 10 mL/g. X-ray fluorescence spectrometer analysis results suggested that the content of sodium oxide was reduced from 4.97% to 0.78% while the contents of SiO₂, Al₂O₃ and Fe₂O₃ were essentially unchanged. X-ray photoelectron spectrometer and X-ray diffraction analysis revealed that hydroxycancrinite in RM was dissolved accompanying sodium released and Mn(II) appeared in the newly-generated form of charmatite in the dealkalized RM. The final dealkalized RM product was demonstrated to be with low soluble heavy metals from its water leaching test. This research provides a sustainable reference of disposal waste with waste for the dealkalization of RM, and the findings of this work demonstrate a new mechanism of replacement Na with Mn in RM dealkalization process.