Abstract Hydrous phases are often considered important hydrogen carriers in the subducted slabs, but their stabilities under the reduced oxidation state have rarely been investigated. Previous studies suggested that metallic iron can be present in the mantle and subducted slabs at depth through disproportionation and serpentinization of silicates. Here we show that hydrous phases in the subducted slabs such as superhydrous phase B, phase E, and phase Egg react with metallic iron to produce less hydrous or anhydrous silicates, oxides, and H2 in the deep mantle. The produced H2 can ascend and react with Fe3+‐bearing silicates in the overlying mantle, leading to the formation of hydrous phases and melts. The high mobility of H2 fluid compared with hydrous species will expedite the deep hydrogen cycle. Meanwhile, the oxidation states of subducted slab and various mantle layers will undergo gradual homogenization as a result of coupled redox dehydrogenation and hydration processes.

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