Argyrodite-type semiconductors, as prototypes of the "phonon-liquid electron crystal" concept, have attracted intensive attention due to their intrinsically low thermal conductivity. Like other members of this family, Cu 8 GeSe 6 undergoes structural phase transition at 330 K from a low-temperature hexagonal phase to a high-temperature hexagonal phase. Here, by employing the entropy engineering strategy through S alloying on the Se site, the phase transition temperature is lowered to below room temperature, so that the stable cubic phase is maintained in the whole operating temperature region. Cubic Cu 8 GeSe 6 exhibits a favorable electronic structure, which leads to improved carrier mobility and electrical performance. In addition, benefiting from the weak chemical bonding between Cu atoms and the [GeSe 6 ] sublattice and the disordered cation occupancy, cubic Cu 8 GeSe 6 possesses an ultralow thermal conductivity, and the sample with a nominal composition of Cu 8 GeSe 4.2 S 1.8 reaches a zT value of 0.5 at 627 K. These results provide insights for suppressing phase transition in argyrodite-type thermoelectrics.