As the reaction product of subducted water and the iron core, iron superoxides which have more oxygen than hematite (Fe₂O₃) has been recently recognized for their potential as a major phase in the D” layer just above the Earth’s core-mantle boundary. Here we report the discovery of new iron superoxide, Fe₄O₇, with a trigonal unit cell of a = 10.100(1) Å, c = 2.634(1) Å at 72 GPa. It was identified by in-situ X-ray diffraction between 40 and 136 GPa upon laser heating mixtures of hematite and goethite (FeOOH) at 1,500–3,400 K. This new superoxide was recoverable back to ambient conditions for ex-situ investigation using transmission electron microscopy. First-principles calculations found Fe₄O₇ to be the most stable phase between Fe₂O₃ and FeO₂ at deep mantle pressures. Like FeO₂, the new Fe₄O₇ is a product of the extraordinary oxidation power of H₂O at high pressure and has low seismic velocities consistent with regions in the D” layer. Unlike FeO₂ which requires water-saturated conditions, Fe₄O₇ can be formed with under-saturated water and is expected to be more ubiquitously at the depth below 1,000 km in Earth’s mantle. Our results also suggest the formation of oxygen-rich Fe₄O₇ may make the deep-mantle redox chemistry more perplex than previously thought.