We report electron-doped AgBi3S5 as a new highperformance nontoxic thermoelectric material. This compound features exceptionally low lattice thermal conductivities of 0.5-0.3 W m(-1) K-1 in the temperature range of 300-800 K, which is ascribed to its unusual vibrational properties: "double rattling" phonon modes associated with Ag and Bi atoms. Chlorine doping at anion sites acts as an efficient electron donor, significantly enhancing the electrical properties of AgBi3S5. In the carrier concentration range (5 X 10(18)-2 X 10(19) cm(-3)) investigated in this study, the trends in Seebeck coefficient can be reasonably understood using a single parabolic band model with the electron effective mass of 0.22 m(e) (m(e) is the free electron mass). Samples of 0.33% Cl-doped AgBi3S5 prepared by spark plasma sintering show a thermoelectric figure of merit of similar to 1.0 at 800 K.