Despite the achievable high thermoelectric properties in SnSe single crystals, the poor mechanical properties and the relatively high cost of synthesis restrict the large scale commercial application of SnSe. Herein, we reported that co-doping with Na and Ag effectively improves the thermoelectric properties of polycrystalline SnSe. Temperature-dependent carrier mobility indicates that the grain boundary scattering is the dominant scattering mechanism near room temperature, giving rise to low electrical conductivity for the polycrystalline SnSe in comparison with that of the single crystal. Co-doping with Na and Ag improves the electrical conductivity of polycrystalline SnSe with a maximum value of 90.1 S cm⁻¹ at 323 K in Na_0.005Ag_0.015Sn_0.98Se, and the electrical conductivity of the (Na, Ag) co-doped samples is higher than that of the single doped samples over the whole temperature range (300–773 K). Considering the relatively high Seebeck coefficient of 335 μV K⁻¹ at 673 K and the minimum thermal conductivity of 0.48 W m⁻¹ K⁻¹ at 773 K, Na_0.005Ag_0.015Sn_0.98Se is observed to have the highest PF and ZT among the series of samples, with values of 0.50 mW cm⁻¹ K⁻² and 0.81 at 773 K, respectively. Its average PF and ZT are 0.43 mW cm⁻¹ K⁻² and 0.37, which is 92% and 68% higher than that of Na_0.02Sn_0.98Se, 40% and 43% higher than that of Ag_0.02Sn_0.98Se, and 304% and 277% higher than that of the previously reported SnSe, respectively.