[en] AgSbTe${}_2$ exhibits superior thermoelectric performance in the mid-temperature region, but its electrical properties are strongly affected by intrinsic defects and secondary phases. Distinctively, the high mobility electrons still play an important role as minority carriers in p-type AgSbTe${}_2$ even below the bipolarization temperature and decrease the Seebeck coefficient. Here, the Al and Se dual alloying can effectively increase the hole concentration by reducing the cation vacancy formation energy and simultaneously reduce the electron concentration by suppressing the n-type Ag${}_2$Te secondary phase are demonstrated, which results in the shift of carrier transport from two-type to one-type as confirmed by Hall measurement. Consequently, through adjusting the ratio of hole and electron conductivity, the average power factor of alloyed sample is enhanced by 70%. Combined with further reduced lattice thermal conductivity resulting from high-density stacking faults and superstructures, a maximum zT of 1.90 and an average zT of 1.42 are obtained in the temperature range of 323 to 623 K in the (AgSbTe${}_2$)${}_{0.98}$(AgAlSe${}_2$)${}_{0.02}$ sample. Finally, based on the finite element analysis modeling results, both single-leg and double-leg thermoelectric devices are fabricated, which show high conversion efficiency of 11.2% and 5.2%, respectively. (© 2024 Wiley‐VCH GmbH)