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Abstract The piezoelectric and ferroelectric applications of heterovalent ternary materials are not well explored. Epitaxial MgSiN₂films are grown at 600 °C on (111)Pt//(001)Al₂O₃substrates by the reactive sputtering method using metallic Mg and Si under the N₂atmosphere. Detailed X‐ray diffraction measurements and transmission electron microscopy observations revealed that the epitaxially grown films on the substrates have a hexagonal wurtzite structure withc‐axis out‐of‐plane orientation. The random occupation of this structure by Mg and Si differs from that of the previously reported structure in which these two cations periodically occupy the cationic sites. However, the lattice spacings closely approximate those that are previously reported, irrespective of the ordering, and they are almost comparable with those of (Al_0.8Sc_0.2)N. The wide bandgap of >5.0 eV in deposited MgSiN₂is compatible with that of AlN and suggests durability against the application of strong external electric fields, possibly to induce polarization switching. In addition, MgSiN₂is shown to have piezoelectric properties with an effectived₃₃value of 2.3 pm V⁻¹for the first time. This work demonstrates the compositional expansion of hexagonal wurtzite to heterovalent ternary nitrides for novel piezoelectric materials, whose ferroelectricity is expected.