The effects of Dy/Nb co–doping on the structural and thermoelectric properties of Sr1−xDyxTi1−yNbyO3−δ (0 ≤ x, y ≤ 0.10) are systematically investigated. Single–doped Sr0.95Dy0.05TiO3−δ and SrTi0.95Nb0.05O3−δ consist of tetragonal (I4/mcm space group) and cubic (Pm3¯m space group) perovskite phases. On the other hand, Dy/Nb co–doped SrTiO3, Sr1−xDyxTi1−yNbyO3−δ, has a tetragonal perovskite phase (I4/mcm space group) since the Dy/Nb co–doping induces a distortion of the TiO6 octahedra and lowers the crystal symmetry. The co–doping effectively increases the electrical conductivity and reduces the thermal conductivity, thus noticeably enhancing the dimensionless figure–of–merit (ZT). The highest ZT value (0.24) is achieved for Sr0.95Dy0.05Ti0.95Nb0.05O3−δ at 600 °C. This ZT value is 41 % and 50 % larger than those of single–doped Sr0.95Dy0.05TiO3−δ (0.17) and SrTi0.95Nb0.05O3−δ (0.16), respectively, at 600 °C.