Argyrodites with a general chemical formula of A 8 BC 6 are known for complex phase transitions, ultralow lattice thermal conductivity, and mixed electronic and ionic conduction. The coexistence of ionic conduction and promising thermoelectric performance have recently been reported in selenide and telluride argyrodites, but scarcely in sulfide argyrodites. Here, the thermoelectric properties of Ag 8 Sn(S 1−x Se x ) 6 are reported. Specifically, Ag 8 SnS 6 exhibits intrinsically ultralow lattice thermal conductivities of 0.61-0.31 W m −1 K −1 over the whole temperature range from 32 to 773 K due to distorted local crystal structure, relatively weak chemical bonding, rattler-like Ag atoms, low-lying optical modes, and dynamic disorder of Ag ions at high temperatures. Se doping shifts the orthorhombic-cubic phase transition from 457 K at x = 0 to 430 K at x = 0.10, thereby expanding the temperature range of the thermoelectrically favored cubic phase. A figure of merit zT value ≈ 0.80 is achieved at 773 K in Ag 8 Sn(S 1−x Se x ) 6 (x = 0.03), the highest zT value reported in sulfide argyrodites. These results fill a knowledge gap of the thermoelectric study of argyrodites and contribute to a comprehensive understanding of the chemical bonding, lattice dynamics, and thermal transport of argyrodites.a potential for practical applications. Yet, it is still a challenge to decouple the adversely correlated {σ, S, κ e } toward high zT values via electronic and phonon band engineering. [3][4][5][6][7][8] Many efforts have been devoted to discover novel TE materials such as cage-like filled skutterudites (e.g., CeFe 4 Sb 12 , CoSb 3 , Sr 8 Ga 16 Ge 30 , etc.), [9][10][11][12] Zintl-phase compounds (e.g., CaZn 2 Sb 2 , Mg 3 Sb 2 , YbMg 2 Bi 2 , etc.), [13][14][15][16] and defective half-Heusler compounds (e.g., Nb 0.8 CoSb, Ti 0.9 NiSb, V 0.9 CoSb, etc.) [17] that possess a long-range order favoring charge carrier mobility while disordered in short range to inhibit heat-carrying phonons in the context of "phonon-glass-electron-crystal". [18] Recently, a class of "liquid-like" TE materials with complex phase transitions, ultralow lattice thermal conductivity, and liquidlike ionic conduction have been reported, Cu 2−x (S, Se, Te), [19][20][21] Ag 2 (S, Se, Te), [22,23] argyrodite, [24][25][26][27][28][29][30][31][32][33][34][35][36] AgCrSe 2 , [37,38] CuAgSe, [39] and AgCuTe [40] to name a few. Argyrodites are good examples of liquid-like TE materials. Argyrodites include 716, 816, and 916 phases in chemistry, with a general chemical formula A 12−n