The thermoelectric properties of the double perovskite-type oxides Sr_2−xK_xFeMoO₆ were investigated in terms of K-doping at the A site of the oxides. The electrical conductivity, σ, of the oxides showed a metallic behavior, decreasing monotonically from ca. 10³ S/cm at room temperature to ca. 10² S/cm at 1250 K. The Seebeck coefficient, S, showed that the oxides are of n-type over the measured temperature range. The absolute value of S increased with increasing temperature. The absolute value of S at 300 K increased up to x = 0.2 and then decreased dramatically for x = 0.3–0.4. The Rietveld refinement of the XRD patterns of the oxides suggested that the increase in orbital degeneracy due to the eased distortion of FeO₆ and MoO₆ octahedra at x = 0.2 was responsible for the |S| maximum at this composition. The power factor, S²σ, of the oxides increased sharply with increasing K-doping level; the value for Sr_1.6K_0.4FeMoO₆ was ca. 4.2 × 10⁻⁴ W/mK², which is the highest among all the samples in this study. The thermal conductivity, κ, of the oxides generally decreased from ca. 3–5 W/mK at room temperature to ca. 2–4 W/mK at 1100 K. Because the power factor of the oxides increased above 900 K, the dimensionless figure of merit, ZT = S²σT/κ, increased dramatically above 900 K. The largest ZT value of 0.24 was observed for Sr_1.6K_0.4FeMoO₆ at 1250 K.