By powder X-ray diffraction, energy dispersive microscopy, Raman spectroscopy, and X-ray crystallographic analysis a secondary phase obtained in the synthesis of sesquilateral yttrium selenide is studied. Results of micro- and spectroscopic techniques show that the composition of crystals is close in the ratio of components to Y:Se ∼1:2, the character of the Raman spectrum clearly indicating the stoichiometric composition of the studied crystal. Positions of strong diffraction reflections obtained from a single crystal are indexed in the unit cell with parameters a = 4.0132(6) Å, b = 4.0061(6) Å, c = 8.2576(19) Å, whose symmetry can be considered tetragonal within the systematic error of the method. Experimental material for the X-ray crystallographic analysis is obtained on an automated Bruker DUO diffractometer with MoK α radiation. The average structure is solved in the tetragonal symmetry (space group P4/nmm, a = 4.0141(6) Å, c = 8.267(2) Å, V = 133.21(4) Å 3 ). In the experimental frames weak superstructural peaks are observed, which evidence a possible increase in unit cell parameters in accordance with the matrix 400/030/002 and the maintenance of the character of structure modulation described for the non-stoichiometric YSe 2−x (x = 0.15) sample.