Structural properties of Tm3+ doped As–S–Ga glasses were investigated using Raman spectroscopy and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy at As K-edge, Ga K-edge and Tm L3-edge. The Raman spectra are interpreted on the basis of comparison with the well-known Raman spectra of As2S3 glasses the structure of which mainly consists of pyramidal [AsS3/2] units interconnected by S–S chains. Incorporation of Ga2S3 in As2S3 glass matrix induced the dissociation of S–S chains in behalf of the formation of tetrahedra [GaS4]. Moreover, introduction of Tm into As–S–Ga glasses forms Tm–S bonds through the further dissociation of [AsS3/2] pyramidal units. The deficiency of S in the glasses is compensated with the formation of As4S4 structural units. The EXAFS results indicate that Ga–S bond distance in the tetrahedra [GaS4] decreases from 2.29Å to 2.26Å with the increase of Tm concentration in the glass. Meanwhile, Tm–S bond distance increases from 2.66Å to 2.71Å approaching to that of crystalline Tm2S3 and simultaneously, the coordination number of Tm gradually decreases from 7 to 4.