Bismuth sulphide is one of the promising thermoelectric material due to its low thermal conductivity, less toxicity, and abundance in nature. This work analyzed an Al-substituted Bi₂S₃ compounds for mid-temperature thermoelectric applications prepared by hydrothermal route. The combination of stacking faults and point defects significantly improved the phonon scattering, resulting in decreased thermal conductivity in (x = 0.10) Al substituted sample with the value of 0.498 Wm⁻¹K⁻¹ at 623 K. Also, it simultaneously contributes to the enhancement in carrier concentration of −2.15 × 10¹⁹ cm⁻³, resulting the increased electrical conductivity of 10,142 Sm^-1 at 563 K. Further, x = 0.025 Al substitution in Bi_2-xAl_xS₃ leads to a high power factor of 338 Wm⁻¹K^-2 and a low phonon thermal conductivity of 0.578 μWm⁻¹K⁻¹, leading to a maximum zT of 0.30 at 623 K.