<p>Polycrystalline SnSe with poor electrical and high thermal transport properties faces great challenges in commercialization, although monocrystalline SnSe boasts a high ZT value. To alleviate this problem, the Ag/In co-doped polycrystalline Sn_{1−2<i>x</i>}Ag_<i>x</i>In_<i>x</i>Se (<i>x</i> = 0.10, 0.15, 0.20) thermoelectric materials are proposed in this work by using vacuum melting/annealing/ball milling process. The co-doping of Ag and In atoms into polycrystalline SnSe matrix effectively modulates the electronic band structure near the Fermi level. At the same time, the introduced endotaxial nanostructure of AgInSe₂ serves as an additional scattering center, effectively scattering lower-energy carriers. This scattering mechanism leads to a substantial increase in the Seebeck coefficient and a marked reduction in lattice thermal conductivity. Notably, the optimized Sn_0.8Ag_0.1In_0.1Se sample exhibited a remarkably low lattice thermal conductivity of 0.29 W m⁻¹ K⁻¹ at 750 K, representing the competitive value reported thus far. The synergistic interplay between the Ag/In co-doping and the endotaxial nanostructure effectively decouples phonon and electron transport in polycrystalline SnSe, leading to a substantial enhancement in its TE performance, providing a novel strategy for the development of practical TE materials with superior performance.</p>