Abstract <p>Referring to the studies reporting recent advances in chalcogenide-based perovskite materials, this study aims to fill the gap in theoretical data for in-depth understanding of monoclinic structure ABS₃ (A = Rb, Cs, Tl) perthioborate materials and explores their potential in photovoltaic and optoelectronic applications. For the first time, their structural, electronic, elastic and optical properties were carefully calculated using the plane wave pseudopotential (PW-PP) method implemented in the DFT framework that are carried out with the CASTEP code. Using the GGA-PBE functional, their optimized crystal lattice calculations reveal that are quite accurate and agree well with available theoretical and experimental measurements. The electronic band calculations reveal that RbBS₃ (CsBS₃) exhibit indirect wide bandgap semiconductor nature 2.32 eV (2.44 eV) and 1.74 eV for TlBS₃, with great potential for use in solar cell or as an absorber. Further, this research provides computational predictions for various mechanical properties that reveal their mechanical stability through Born and Pugh ratio criteria, ductile, ionic character, lower shear resistance than compression and high elastic anisotropy. Their bulk and young’s modulus are calculated using a 3D plotting representation. In addition, RbBS₃ exhibits higher thermal conductivity, due to its significantly higher Debye temperature θ_D. Moreover, several optical properties were calculated that showed high UV absorption coefficient up to (~10⁵ cm^–1), optical conductivity peaks reach (~10¹⁵ s^–1) in UV-Visible range for three studied compounds and significantly lower reflectivity (10%) seen by RbBS₃ (TlBS₃), than CsBS₃ (18%) across a wide energy spectrum, making them the most promising candidate for photovoltaic and optoelectronic applications.</p>