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La₃Te₄-based rare-earth telluride is a kind of n-type high-temperature thermoelectric (TE) material with an operational temperature of up to 1273 K, which is a promising candidate for thermoelectric generators. In this work, the Sm substitution in La_3−<i>x</i>Sm<i>_x</i>Te₄/Ni composites is reported. The electrical transport property of La_3−<i>x</i>Sm<i>_x</i>Te₄ is modified by reducing carrier concentration due to the substitution of Sm²⁺ for La³⁺. The electric thermal conductivity decreases by 90% due to carrier concentration reduction, which mainly contributes to a reduction in total thermal conductivity. Lattice thermal conductivity also decreases by point-defect scattering by Sm doping. Meanwhile, based on our previous study, compositing nickel improves the thermal stability of the La_{3 − <i>x</i>}Sm<i>_x</i>Te₄ matrix. Finally, combined with carrier concentration optimization and the decreased thermal conductivity, a maximum <i>zT</i> of 1.1 at 1273 K and an average <i>zT</i>_ave value of 0.8 over 600 K–1273 K were achieved in La_2.315Sm_0.685Te₄/10 vol.% Ni composite, which is among the highest TE performance reported in La₃Te₄ compounds.