Cu₃SbSe₄ is a potential p-type thermoelectric material, distinguished by its earth-abundant, inexpensive, innocuous, and environmentally friendly components. Nonetheless, the thermoelectric performance is poor and remains subpar. Herein, the electrical and thermal transport properties of Cu₃SbSe₄ were synergistically optimized by S alloying. Firstly, S alloying widened the band gap, effectively alleviating the bipolar effect. Additionally, the substitution of S in the lattice significantly increased the carrier effective mass, leading to a large Seebeck coefficient of ~730 μVK⁻¹. Moreover, S alloying yielded point defect and Umklapp scattering to significantly depress the lattice thermal conductivity, and thus brought about an ultralow κ_lat ~0.50 Wm⁻¹K⁻¹ at 673 K in the solid solution. Consequently, multiple effects induced by S alloying enhanced the thermoelectric performance of the Cu₃SbSe₄-Cu₃SbS₄ solid solution, resulting in a maximum ZT value of ~0.72 at 673 K for the Cu₃SbSe_2.8S_1.2 sample, which was ~44% higher than that of pristine Cu₃SbSe₄. This work offers direction on improving the comprehensive TE in solid solutions via elemental alloying.