<p>Sb₂Te₃ alloys are well-known <i>p</i>-type thermoelectric material exhibiting strong thermoelectric performance in the mid-temperature range, and Sb₂Te₃ composition serves as the parent compound for many of the highest-performing thermoelectric materials in the low-to-mid temperature range, including (Bi, Sb)₂Te₃ and In-doped Sb₂Te₃. This study systematically investigates the thermoelectric transport properties of Ni-doped Sb₂Te₃ alloys with nominal compositions of (Sb_1 − xNi_x)₂Te₃ (<i>x</i> = 0, 0.015, 0.03, 0.045, and 0.06). Ni substitution at Sb sites effectively increased the Hall carrier concentration from 5.41 × 10¹⁹ cm^− 3 (<i>x</i> = 0) to 8.41 × 10¹⁹ cm⁻³ (<i>x</i> = 0.06). Although the density-of-states effective mass (<i>m</i>_d^*) increased with Ni content, a substantial reduction in non-degenerate mobility led to an overall 20% decrease in the power factor. Regarding thermal transport, the reduction in lattice thermal conductivity was offset by a significant increase in electronic thermal conductivity, resulting in a net increase in total thermal conductivity. Consequently, the maximum thermoelectric figure of merit (<i>zT</i>) decreased from that of the pristine sample, with the lowest <i>zT</i> of 0.31 observed for the <i>x</i> = 0.06 sample at 300&#xa0;K, a ~ 17% reduction. However, analysis based on the single parabolic band (SPB) model revealed that the enhanced <i>m</i>_d^* at higher doping levels holds significant potential. By optimizing the carrier concentration to ~ 1.13 × 10¹⁹ cm^− 3, a peak <i>zT</i> of 0.67 was predicted for the <i>x</i> = 0.06 composition, suggesting a clear pathway for future performance enhancement.</p> Graphical Abstract <p></p>