<p>Because of their low toxicity and abundance, Cu₂SnS₃-based thermoelectric materials are gaining a lot of attention as environmentally friendly options for waste heat recovery. In this work, pristine and indium-doped Cu₂SnS₃ samples were synthesised to examine the defect engineering influence on thermoelectric performance. The introduction of indium leads to lattice distortion and cation disorder, thereby improving the scattering of phonon and significantly decreasing the thermal conductivity of the lattice. The ideally doped sample had an ultralow thermal conductivity of ~ 0.39 W m⁻¹&#xa0;K⁻¹ at 303&#xa0;K. Additionally, the addition of indium increases the concentration of carriers, which improves electrical conductivity. The interplay between enhanced charge transport and reduced thermal conductivity yields a maximum figure of merit (<i>zT</i>) of 0.046 at 573&#xa0;K. Although the achieved zT is modest, the results clearly demonstrate that indium-induced defect engineering is an effective approach for tuning thermal and electrical transport in Cu₂SnS₃. The potential of this material system for more optimization toward mid-temperature thermoelectric applications is highlighted in this study.</p>