Er³⁺ clustering phenomenon in Ga-Ge-S chalcogenide system is studied using Raman spectroscopy. The Raman spectra from 10 to 500 cm ^-1 for glasses (100-y)[15Ga₂S₃-85GeS ₂]-yEr₂S₃ (y=0.08-5.00 mol. %) have been analyzed. To reveal the influence of the chemical composition on the glass structure the intensity of the peak corresponding to Ge-Ge (Ga-Ga) homopolar bonds has been examined. The peak intensity increase with Er₂S ₃ concentration change in the region 0<C(Er₂S ₃)<2 mol. % has been interpreted in terms of the sulphur deficiency in the glass resulting in the formation of S₃Ge-GeS ₃ (S₃Ga-GaS₃) structural units. The further increase in concentration beyond 2 mol. % reduces the sulphur deficiency, which can be attributed to the formation of the ternary compound Er ₃GaS₆. The structural units Er₃GaS₆ contain a large mol. fraction of Er³⁺ or, in other words, Er ³⁺ clusters. The data obtained from the low-frequency Raman spectra (boson band) indicate strong variations of the medium-range order (MRO) in the glasses induced by Er³⁺. The observed behavior of the MRO size (the correlation length) with increasing of Er₂S₃ concentration provides for additional evidence of the Er³⁺ clustering.