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Forming co-alloying solid solutions has long been considered as an effective strategy for improving thermoelectric performance. Herein, the dense Cu_2−<i>x</i>(MnFeNi)<i>_x</i>Se (<i>x</i> = 0–0.09) with intrinsically low thermal conductivity was prepared by a melting-ball milling-hot pressing process. The influences of nanostructure and compositional gradient on the microstructure and thermoelectric properties of Cu₂Se were evaluated. It was found that the thermal conductivity decreased from 1.54 Wm⁻¹K⁻¹ to 0.64 Wm⁻¹K⁻¹ at 300 K via the phonon scattering mechanisms caused by atomic disorder and nano defects. The maximum <i>zT</i> value for the Cu_1.91(MnFeNi)_0.09Se sample was 1.08 at 750 K, which was about 27% higher than that of a pristine sample.