The equilibrium alloys of the FeGa₂S₄–FeIn₂S₄ system were studied by the method of powder X-ray diffraction. The formation of limited substitutional solid solutions based on the initial ternary compounds and the existence of a quaternary FeGaInS₄ compound were established. Based on powder diffraction data, by Rietveld method the crystal structure of the FeGaInS₄ compound was refined. It was shown that compound crystallizes in the trigonal system (Sp.gr.P-3m1) with lattice parameters: a = 3.7765(1), c = 12.2257(3) Å, V = 151.00(1) ų.