Polycrystalline olivine-type sulfides, Mn₂SiS₄, Mn₂GeS₄, Fe₂SiS₄, and Fe₂GeS₄, are prepared by solid-state reactions and spark plasma sintering. Among these sulfides, only the electrical conductivities and Seebeck coefficients of Fe₂SiS₄ and Fe₂GeS₄ are measurable, owing to their narrower energy gaps compared with those of Mn₂SiS₄ and Mn₂GeS₄. Although Fe₂SiS₄ and Fe₂GeS₄ exhibit low electrical conductivities, their Seebeck coefficients and thermal conductivities are relatively high and low, respectively, demonstrating their potential as good thermoelectric materials. The highest zT value, 0.008, is obtained for Fe₂GeS₄ at 700 K, which can be enhanced by hole carrier doping and heavy elemental partial substitution.