Halide perovskite semiconductors exhibit ultralow thermal conductivities, making them potentially suitable for thermoelectric applications. Nevertheless, the thermoelectric properties of the prototypical halide perovskite of CH₃NH₃PbI₃ have been limited with a very low dimensionless figure of merit (ZT) and a narrow operating temperature window, which are attributed to its poor electronic conductivity and unstable hybrid organic–inorganic composition, respectively. Here, we report the bulk synthesis of a stable, all-inorganic halide perovskite of CsSn_0.8Ge_0.2I₃ as a new thermoelectric material, which shows a 10 order of magnitude enhancement in ZT compared with that of CH₃NH₃PbI₃ and an operating temperature as high as 473 K. Importantly, this CsSn_0.8Ge_0.2I₃ perovskite is also Pb-free in the composition, attesting its high potential as an environmentally friendly candidate material for future thermoelectrics.