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Thermoelectrics (TEs) have been considered a sustainable and eco-friendly energy technology. However, due to limitations in energy conversion efficiency, TE devices have not yet been widely adopted. Here, we proposed a class of TE materials, trigonal XPt2Se3 (X = K, Rb), with the same crystal structure as Bi2Te3. At room temperature, with quartic anharmonicity correction, the lattice thermal conductivity (κL) of KPt2Se3 and RbPt2Se3 in the x-direction is only 0.57 and 0.46 W m−1 K−1, respectively. The ultralow κL arises from their layered structure, strong lattice anharmonicity, weak bonding nature, rattling motion of guest alkali metal atoms, and large scattering space. Simultaneously, the large density of states contributes to large power factors. At 800 K, both under n-type and p-type doping, KPt2Se3 exhibits ZT values that can exceed 4 in specific directions, while RbPt2Se3's ZT values can surpass 3, which is significantly higher than traditional TE materials. Our research not only elucidates that the layered trigonal crystals XPt2Se3 (X = K, Rb) represent a category of potential TE materials with ultralow κL and high TE performance but also provides directions for exploring TE materials. [ABSTRACT FROM AUTHOR]