SrCoO 3− δ outperforms as cathode material in solid-oxide fuel cells (SOFC) when the three-dimensional (3C-type) perovskite structure is stabilized by the inclusion of highly-charged transition-metal ions at the octahedral positions. In a previous work we studied the Nb incorporation at the Co positions in the SrCo 1− x Nb x O 3− δ system, in which the stabilization of a tetragonal P4 / mmm perovskite superstructure was described for the x = 0.05 composition. In the present study we extend this investigation to the x = 0.10–0.15 range, also observing the formation of the tetragonal P4 / mmm structure instead of the unwanted hexagonal phase corresponding to the 2H polytype. We also investigated the effect of Nb 5+ doping on the thermal, electrical, and electrochemical properties of SrCo 1− x Nb x O 3− δ ( x = 0.1 and 0.15) perovskite oxides performing as cathodes in SOFC. In comparison with the undoped hexagonal SrCoO 3− δ phase, the resulting compounds present high thermal stability and an increase of the electrical conductivity. The single-cell tests for these compositions ( x = 0.10 and 0.15) with La 0.8 Sr 0.2 Ga 0.83 Mg 0.17 O 3− δ (LSGM) as electrolyte and SrMo 0.8 Fe 0.2 CoO 3− δ as anode gave maximum power densities of 693 and 550 mW∙cm −2 at 850 °C respectively, using pure H 2 as fuel and air as oxidant.