Mg2P2O7 has two known polymorphs, i.e., α- and β-phase, at low and high temperatures, respectively. Here, we report a new polymorph of Mg2P2O7, γ-Mg2P2O7, obtained from a high-pressure and high-temperature experiment at 8 GPa and 1473 K. Single-crystal X-ray diffraction shows that γ-Mg2P2O7 is monoclinic with the space group of P21/c and the unit–cell parameters a = 4.5080(1), b = 9.9450(2), c = 5.2358(1) Å, β = 97.93(1)°, V = 232.48(1) Å3, and Z = 2. The crystal structure of γ-Mg2P2O7 is characterized by undulating alternant layers made of MgO6 octahedra and [P2O7]4– tetrahedral dimers. Raman spectrum of γ-Mg2P2O7 was analyzed and assigned. γ-Mg2P2O7 may serve as a potential host for phosphors.