As thermoelectric generators can convert waste heat into electricity, they play an important role in energy harvesting. The metal chalcogenide AgBiSe2 is one of the high-performance thermoelectric materials with low lattice thermal conductivity (κlat), but it exhibits temperature-dependent crystal structural transitions from hexagonal to rhombohedral, and finally a cubic phase as the temperature rises. The high figure-of-merit ZT is obtained only for the high-temperature cubic phase. In this study, we utilized the high-entropy-alloy (HEA) concept for AgBiSe2 to stabilize the cubic phase throughout the entire temperature range with enhanced thermoelectric performance. We synthesized high-entropy-type AgBiSe2−2xSxTex bulk polycrystals and realized the stabilization of the cubic phase from room temperature to 800K for x ≥ 0.6. The ultra-low κlat of 0.30 Wm-1K-1 and the high peak ZT ~ 0.9 at around 750K were realized for cubic AgBiSe2−2xSxTex without carrier tuning. In addition, the average ZT value of x = 0.6 and 0.7 for the temperature range of 360–750K increased to 0.38 and 0.40, respectively, which are comparable to the highest previously reported values.