The crystal structure and chemical composition of a crystal of Mg(Mg,Cr,Si)2O4 post-spinel phase synthesized in the model system MgCr2O4–Mg2SiO4 at 16 GPa and 1600 °C have been investigated. The compound was found to crystallize with a distorted orthorhombic calcium-titanate (CaTi2O4) structure type, space group Cmc21, with lattice parameters a = 2.8482(1), b = 9.4592(5), c = 9.6353(5) Å, V = 259.59(1) Å3, and Z = 4. The structure was refined to R1 = 0.018 using 345 independent reflections. The loss of the inversion center is due to the ordering of cations at the octahedral sites: Cr is mainly hosted at the M1 site, whereas Si at the M2 site. Such an ordered distribution induces a distortion thus provoking a change in coordination of Mg, which becomes sevenfold-coordinated instead of the usual eightfold coordination observed in post-spinel phases. Electron microprobe analysis gave the Mg[(Cr0.792Mg0.208)(Si0.603Mg0.397)]O4 stoichiometry for the studied phase. The successful synthesis of this phase can provide new constraints on thermobarometry of wadsleyite/ringwoodite-bearing assemblages.