To synthesize alloys of the Se–SrGa₂Se₄ system, the SrGa₂Se₄ compound was first obtained from the binary compounds SrSe and Ga₂Se₃ via the ampoule method in the temperature range of 1000–1200°C. Subsequently, alloys were synthesized based on Se and SrGa₂Se₄. Single crystals of the Sr thiogallate were grown using the Bridgman method. For the first time, the phase equilibria of the Se–SrGa₂Se₄ system were studied through differential thermal analysis (DTA), X-ray phase analysis (XRD), microstructural analysis (MSA), as well as density and microhardness measurements, and the T–x phase diagram was constructed. A eutectic reaction was identified within the Ga₂Se₃ concentration range. The SrGa₂Se₄ compound was found to melt congruently at 1110°C, crystallizing in the orthorhombic system, with lattice parameters determined as a = 21.56 Å, b = 21.45 Å, and c = 12.75 Å. The physical properties were determined on a solid solution based on SrGa₂Se₄ containing 6 mol% Se. The SrGa₂Se₄ compound has potential applications in optoelectronics, lasers, radiation detectors, dosimetric systems, photovoltaic materials, solar panels, and can also be used in the synthesis of phosphor materials when activated with rare earth elements.