Thermoelectric technology allows conversion between heat and electricity. Many good thermoelectric materials contain rare or toxic elements, so developing low-cost and high-performance thermoelectric materials is warranted. Here, we report the temperature-dependent interplay of three separate electronic bands in hole-doped tin sulfide (SnS) crystals. This behavior leads to synergistic optimization between effective mass (m*) and carrier mobility (mu) and can be boosted through introducing selenium (Se). This enhanced the power factor from similar to 30 to similar to 53 microwatts per centimeter per square kelvin (mu W cm(-1) K-2 at 300 K). while lowering the thermal conductivity after Se alloying. As a result, we obtained a maximum figure of merit ZT (ZT(max)) of similar to 1.6 at 873 K and an average ZT (ZI(ave)) of similar to 125 at 300 to 873 K in SnS0.91Se0.09 crystals. Our strategy for band manipulation offers a different route for optimizing thermoelectric performance. The high-performance SnS crystals represent an important step toward low-cost, Earth-abundant, and environmentally friendly thermoelectrics.