High thermoelectric efficiency has been reported in several antiperovskites, but strongly unbalanced n- and p-type thermoelectric performances still limit practical device integration. Here, we show that ${\mathrm{Ag}}_3\mathrm{SX}$ (X = Cl, Br, I) antiperovskites exhibit simultaneously favorable p- and n-type transport, and that halogen substitution further improves this ambipolar behavior through coupled lattice and electronic mechanisms. On the lattice side, heavier halogens soften the Ag-X framework and strengthen anharmonic phonon scattering, which markedly lowers the lattice thermal conductivity and helps drive the materials toward a glasslike transport regime. On the electronic side, halogen substitution modifies the relative contributions and hybridization of Ag-4d and chalcogen/halogen p states near the valence-band edge; together with the stronger spin–orbit coupling in the iodide, this leads to enhanced valence-band degeneracy while retaining sufficient band dispersion. As a result, ${\mathrm{Ag}}_3\mathrm{SI}$ exhibits ultralow lattice thermal conductivity (0.29 W/m K) and high carrier mobility (10–200 $\mathrm{c}{\mathrm{m}}^2/\mathrm{V}\mathrm{s}$), achieving a maximum power factor of 2.5 $\mathrm{mW}/\mathrm{m}{\mathrm{K}}^2$. Under optimal doping, $\mathit{ZT}$ at 800 K reaches ≈2.42 for p-type and ≈1.87 for n-type carriers. These results identify ${\mathrm{Ag}}_3\mathrm{SX}$ compounds as promising ambipolar thermoelectric materials and highlight how lattice anharmonicity and band-edge reconstruction can be combined to optimize heat and charge transport.

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• Electronic structure
• Lattice thermal conductivity
• Thermoelectrics
• Perovskites
• Density functional calculations