An ultralow-temperature hydrothermal strategy was for the first time reported to synthesize spinel-type zinc manganese oxide (ZnMn2O4) nanocrystals at 70°C. The synthetic parameters including hydrothermal temperature, zinc or manganese source, hydrothermal time, and NaOH concentration were systematically investigated. The structure and morphology characterization revealed that the resulting Zn–Mn spinels are tetragonal-phase, and have well-defined crystalline structure with the size of 10–20nm. Furthermore, a defect spinel of λ-MnO2 nanocrystal (10–20nm) was directly synthesized from ZnMn2O4 nanocrystal upon acid leaching. Finally, a redox mechanism for the formation of λ-MnO2was discussed.