Bi_0.88−xZn_xSb_0.12 alloys with x = 0.00, 0.05, 0.10, and 0.15 were prepared using hydrothermal synthesis in combination with evacuating-and-encapsulating sintering. The effects of partial Zn substitution for Bi and different sintering temperatures on the thermoelectric properties of Bi_0.88−xZn_xSb_0.12 alloys were investigated between 25 K and 425 K. Both the electrical conductivity and absolute thermopower are enhanced for the set of alloys sintered at 250 °C. The maximum power factor of 57.60 μW cm⁻¹ K⁻² is attained for the x = 0.05 alloy sintered at 250 °C. As compared with Zn-free Bi_0.88Sb_0.12, both the total thermal conductivity and lattice component are reduced upon Zn doping. Bipolar conduction is observed in both electronic and thermal transport. The maximum zT of 0.47 is attained at 275 K for the x = 0.05 alloy sintered at 250 °C.