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The formation mechanism, phase stability, optical and magnetodielectric properties of new cubic MgMnO3 bulk and nano-crystals are reported. The diffuse reflectance spectroscopy of this compound shows a ligand-to-metal charge transfer transition p[O2−]→eg[Mg2+] at 4 eV and the lowest spin allowed metal-to-metal transition at 2.43 eV. In bulk MgMnO3 with the defect cubic structure [Mg2+][Mg2+1/3Mn4+4/3□1/3]O4 (Seehra et al. Appl. Phys. Lett. 97, 112507 (2010)10.1063/1.3489094) a clear change in the dielectric constant (ɛ′) is observed on cooling through its blocking temperature TB ∼17.5 K. This dielectric anomaly is magnetic field independent even at 14 kOe. A clear evidence of the magneto-dielectric effect is observed for T < TB. Possible origin of the magneto-dielectric coupling has been discussed.