Lead-free halide perovskites are promising candidates for optoelectronic devices due to their superior stability and reduced environmental impact. In this study, the structural, mechanical, electronic, and excitonic optical properties of cubic Ba₃AsCl₃ were explored using first-principles DFT and MBPT calculations. Structural optimization confirmed that Ba₃AsCl3 with formation energy of −2.76 Ry is thermodynamically stable, consistent with previous theoretical calculations. Mechanical stability is verified by elastic constants that satisfy the Born-Huang criteria. Voigt-Reuss-Hill averaging yields bulk modulus of 22.90–24.01 GPa, Young’s modulus of 6.41-6.91 GPa, shear modulus of 2.23–2.33 GPa, and Poisson’s ratio of 0.56-0.68, characterizing Ba₃AsCl₃ as mechanically compliant with a pronounced ductile character. The high Pugh’s ratios of 9.88-10.48 and elastic anisotropy factors of 0.278-0.401 further confirm its ductile and strongly anisotropic mechanical behaviour. Electronic structure analysis shows that standard DFT underestimates the bandgap, giving a direct band gap of 0.98 eV with PBE and 0.56 eV with LDA. Quasiparticle corrections via G₀W₀ increase the gap to 1.79 eV while preserving its direct nature at Γ, highlighting the necessity of many-body effects. Excitonic and optical properties were computed by solving the Bethe-Salpeter equation on top of G₀W₀. Inclusion of electron-hole interactions induces a redshift and a pronounced excitonic peak at 1.42 eV, yielding an exciton binding energy of 0.37 eV. The G₀W₀+BSE framework predicts significant visible-range absorption, a static dielectric constant of 4.31, a refractive index of 2.08, and low reflectivity, underscoring strong light–matter coupling. These results establish Ba₃AsCl3 as a promising lead-free semiconductor for photovoltaic and optoelectronic devices.