The compound MnGaInSe₄ was synthesized from the ternary compounds MnGa₂Se₄ and MnIn₂Se₄ by melting in conditions of vacuum. Its crystalline structure has been made precise on the basis of powder diffraction data by the Rietveld method. It is shown that the compound crystallizes in the trigonal syngony with lattice parameters: a = 3.9812 (1) Å, c = 12.9904 (4) Å, V = 178.32 (1) ų, space group P-3m1. It was also found that the compounds MnGaInSe₄ and ZnIn₂S₄ are isostructural.