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Oxide thermoelectric ceramics attract extensive research interest due to their excellent thermal stability, non‐toxicity, cost‐effectiveness, and earth‐abundant constituent elements. In the present work, the authors study the effect of lanthanum (La) and samarium (Sm) co‐doping on the structural, microstructural, and thermoelectric properties of CaMnO3 ceramics. The X‐ray diffraction (XRD) studies show an increase in unit cell volume with an increase in Sm concentration. A shift in Raman modes is observed, confirming distortions in MnO6 octahedra with the substitution of rare‐earth elements. The co‐doping strategy leads to a two‐order increment in electrical conductivity and reduction in thermopower as expected due to the increase in Mn3+ and charge carrier concentration. The highest electrical conductivity value attained is ≈200 S cm−1 at around 570 K for La0.025Sm0.1Ca0.875MnO3. The maximum value of thermoelectric power factor achieved is 194 μW m–1 K–2, which is two orders higher than that of the undoped CaMnO3. [ABSTRACT FROM AUTHOR]