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• Growth of large-sized Ge 1– x In x Bi 2 Te 4 single crystals by Bridgman method. • Ultralow κ L of GeBi 2 Te 4 driven by cation disorder and low phonon velocity. • Resonant level introduced by In-doping greatly improves electrical performance. • Achieving zT ave of 0.36 in the range of 323–573 K for Ge 0.97 In 0.03 Bi 2 Te 4 sample. Resonant level, the impurity level located inside the valence/conduction band, is an effective strategy for decoupling the electrical conductivity and Seebeck coefficient and enhancing the thermoelectric power factor. In this work, we show that In doping induces resonant level and enhances the thermoelectric performance of GeBi 2 Te 4 , a promising n-type thermoelectric material with layered structure. Single crystals grown by Bridgman method were used in this study to take the advantage of its anisotropic transport properties. An ultralow lattice thermal conductivity of ∼0.51 W m−1 K−1 at 323 K was found along the c -axis of GeBi 2 Te 4 , driven by intrinsic cation disorder and the low phonon group velocity from the relatively weak chemical bonds as well as the strong lattice anharmonicity caused by hierarchical bond orders. Both enhanced electrical conductivity and Seebeck coefficient were observed in the In-doped samples, indicating that In-doping not only increases the carrier concentration, but also introduces the resonant level that was further confirmed by the first-principles calculation. A maximum zT of ∼0.42 at 523 K and an average zT of 0.36 over 323 to 573 K were achieved in Ge 0.97 In 0.03 Bi 2 Te 4 crystal sample, ∼100% and ∼89% enhancement compared to those of pristine single crystal sample, respectively. This study demonstrates another example of resonant level for thermoelectric performance enhancement, which may be applied to other related materials. [ABSTRACT FROM AUTHOR]