AgBiS is an environmentally friendly n-type thermoelectric material composed of earth-abundant and nontoxic elements. It has a complex monoclinic structure with distorted NaCl-type fragments, which provide its intrinsically low thermal conductivity. However, poor electrical properties limit its overall performance. Configurational entropy engineering is an effective method to enhance thermoelectric properties. With the increase of configurational entropy, phonon point defect scattering is amplified, yielding lower lattice thermal conductivity, while the structure symmetry can also be improved, which leads to the enhanced electrical transport property. In this study, we combine carrier modulation and entropy engineering, utilizing melting-annealing and spark plasma sintering, to synthesize a series of AgBi(SeS) bulks. Se substitution effectively increases the configurational entropy and thus dramatically decreases the thermal conductivity. Moreover, anion deficiency modulation effectively optimizes the carrier concentration and the electrical transport properties. Due to a power factor of 2.7 μW/(cm·K) and a low thermal conductivity of 0.45 W/(m·K) at 723 K, the AgBi(SeS) sample possesses the highest of 0.42 at 723 K, nearly double the value of AgBiS or pristine AgBiS. Our work demonstrates that apart from carrier optimization, entropy engineering opens a new avenue for enhancing the thermoelectric properties of a given material.