Recently, extremely low lattice thermal conductivity value has been reported for alkali based telluride material, BaIn 2 Te 4 . The value is comparable with low-thermal conductivity metal chalcogenides and the glass limit is highly intriguing. Therefore, to shed light on this issue, we performed first-principles phonon thermal transport calculations. We predicted highly anisotropic lattice thermal conductivity along different directions by the solution of linearized phonon Boltzmann transport equation. More importantly, we determined several different factors as the main sources of the predicted ultralow lattice thermal conductivity of this crystal, such as the strong interactions between low-frequency optical phonons and acoustic phonons, small phonon group velocities, and lattice anharmonicity indicated by large negative mode Grüneisen parameters. Along with thermal transport calculations, we also investigated the electronic transport properties by accurately calculating the scattering mechanisms, namely, acoustic deformation potential, ionized impurity, and polar optical scatterings. The inclusion of spin-orbit coupling (SOC) for electronic structure is found to be strongly affect the p-type Seebeck coefficients. Finally, we calculated the thermoelectric properties accurately and the optimal ZT value of p-type doping, which originated from high Seebeck coefficients, was predicted to exceed unity after 700 K and have a direction averaged value of 1.63 (1.76 at y-direction) at 1000 K around 2 × 10 20 cm −3 hole concentration. For the n-type doping, the ZT around 3.2 × 10 19 cm −3 concentration was predicted to be as direction averaged value of 1.40 (1.76 at z-direction) at 1000 K, mostly originating from its high electron mobility.With the experimental evidence of high thermal stability, we showed that the BaIn 2 Te 4 compound has the potential for being a promising mid-to-high temperature thermoelectric material for both p-type and n-type systems with appropriate doping.