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Rock-salt-structured compounds like lead chalcogenides are promising thermoelectrics, as their high symmetry, strong anharmonicity, and favorable phase behavior collectively lead to high performance by enabling large power factors and ultralow thermal conductivity. Here, we report LiMnSbTe, a new rock-salt semiconductor stabilized through targeted chemical design by combining hexagonal MnTe with cubic LiSbTe. Embedded in the high-symmetry matrix, van der Waals-like gaps form due to SbTe nanoscale segregation, which acts as effective phonon-scattering centers, leading to a low lattice thermal conductivity of 0.37 WmK at 873 K with alloy scattering from disordered cations. The ordered local structure of SbTe-type vdW-like gaps and the cross-gap interaction facilitate the carrier transport. Aided by energy-converged valence bands and a paramagnon drag effect, high Seebeck coefficients and enhanced power factor can be achieved, leading to a high ZT of 1.2 at 873 K. Furthermore, introducing Mn deficiency increases ZT to 1.5, highlighting the potential for higher performance through optimized doping or alloying. A segmented single-leg thermoelectric module achieves an output power density of 0.52 Wcm and an efficiency of 8.7% under ∆T of 478 K, further demonstrating its promising thermoelectric applications.