Polycrystalline bulk ingot of Sb_1.7Mn_0.3Se₃ was synthesized. Thin films were prepared by thermal evaporation of the pre-reacted material on glass substrates. The elemental analysis of both bulk and the as-deposited films revealed the chemical formula Sb_1.69Mn_0.29Se_3.02 for the bulk material, while Sb_1.67Mn_0.23Se_3.1 for the as-deposited films. X-ray diffraction (XRD) studies on the starting material revealed that the prepared ingot material has predominant and minor secondary isostructural phases of the orthorhombic type structure with the space group Pbnm (62). On the other hand, XRD studies on the as-deposited and annealed films, at T_a = 373, 423 and 473 K, revealed an amorphous-to-crystalline phase transition. The as-deposited films are amorphous in nature and the films annealed at T_a = 373 K are partially crystallized, while those annealed at T_a = 423 and 473 K are crystalline. Besides, both the partially crystallized and crystalline films were found to be a single phase of an orthorhombic type structure, the same as that of the predominant phase in the bulk material. The determined unit-cell lattice parameters of both the predominant phase in the bulk material and the crystalline films were found to be in suitable agreement. The optical constants of the investigated films were determined from optical transmittance data using the Swanepoel method. The dispersion parameters were determined from the analysis of the refractive index. The analysis of the optical absorption spectra revealed that the crystalline films exhibited both direct and indirect energy gaps, while the amorphous and partially crystallized films showed only a non-direct energy gap with a relatively higher Urbach's tail. The effect of annealing on the dispersion parameters as well as the optical transition shows that ε_∞ (= n² − 1) increases from 8.7 to 11.8, while the indirect band gap decreases from 1.37 to 1.14 eV, by annealing from room temperature to 473 K.