Layered nickelates, as representatives of Ruddlesden-Popper phases, are widely used in electrochemical, catalytic and other applications due to their advanced transport and thermomechanical characteristics. In this work, La2–xSmxNiO4+δ (x = 0.0 – 1.0) complex oxides are obtained and then comprehensively studied as potential oxygen electrode materials for solid oxide fuel and electrolysis cells (SOFCs and SOECs). These materials are synthesized by a glycerol-nitrate combustion method. The obtained compounds are single-phase up to x = 0.8 and possess a tetragonal Ruddlesden-Popper structure (I4/mmm sp. gr.). A combination of complementary techniques, including reverse dichromatometric titration, thermogravimetric and isotope exchange methods has been used to demonstrate that partial Sm substitution results in an increase in the over-stoichiometric oxygen content in the materials and oxygen transport kinetics. These materials are found to be mixed ionic-electronic conductors with predominantly p-type electronic conductivity exceeding 110 S/cm at 450 °C in air. The Sm-containing nickelates exhibit improved thermomechanical compatibility and show no chemical interaction with CeO2-based solid electrolytes compared to the undoped La2NiO4+δ. The electrochemical activity of bilayer electrodes consisting of the La2–xSmxNiO4+δ functional layers and LaNi0.6Fe0.4O3–δ-based collectors are studied in air at 600 – 850 °C in the Ce0.8Sm0.2O1.9-based symmetrical cells. Electrochemical impedance spectroscopy study and following distribution of relaxation times analysis indicate a positive impact of the increased oxygen mobility in the La2–xSmxNiO4+δ materials on the overall electrode performance. Due to their superior electrode characteristics compared to La2NiO4+δ, La2–xSmxNiO4+δ with a moderate dopant content (x = 0.2 – 0.3) can be recommended for use as oxygen electrodes for both SOFCs and SOECs.