The electron transport properties of 809 sulfides have been investigated using density functional theory (DFT) calculations in the relaxation time approximation, and a material design rule established for high-performance sulfide thermoelectric (TE) materials. Benchmark electron transport calculations were performed for Cu12Sb4S13 and Cu26V2Ge6S32, revealing that the ratio of the scattering probability of electrons and phonons (κlatτel −1) was constant at about 2 × 1014 W K−1 m−1 s−1. The calculated thermopower S dependence of the theoretical dimensionless figure of merit ZTDFT of the 809 sulfides showed a maximum at 140 μV K−1 to 170 μV K−1. Under the assumption of constant κlatτel −1 of 2 × 1014 W K−1 m−1 s−1 and constant group velocity v of electrons, a slope of the density of states of 8.6 states eV−2 to 10 states eV−2 is suitable for high-ZT sulfide TE materials. The Lorenz number L dependence of ZTDFT for the 809 sulfides showed a maximum at L of approximately 2.45 × 10−8 V2 K−2. This result demonstrates that the potential of high-ZT sulfide materials is highest when the electron thermal conductivity κel of the symmetric band is equal to that of the asymmetric band.