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Two-dimensional (2D) thermoelectric (TE) materials have been widely developed; however, some 2D materials exhibit isotropic phonon, electron transport properties, and poor TE performance, which limit their application scope. Thus, exploring excellent anisotropic and ultrahigh-performance TE materials are very warranted. Herein, we first investigate the phonon thermal and TE properties of a novel 2D-connectivity ternary compound named Ga₂I₂S₂ . This paper comprehensively studies the phonon dispersion, phonon anharmonicity, lattice thermal conductivity, electronic structure, carrier mobility, Seebeck coefficient, electrical conductivity, and the dimensionless figure of merit (ZT) versus carrier concentration for 2D Ga₂I₂S₂ . We conclude that the in-plane lattice thermal conductivities of Ga₂I₂S₂ at room temperature (300 K) are found to be 1.55 W mK⁻¹ in the X-axis direction (xx-direction) and 3.82 W mK⁻¹ in the Y-axis direction (yy-direction), which means its anisotropy ratio reaches 1.46. Simultaneously, the TE performance of p-type and n-type doping 2D Ga₂I₂S₂ also shows significant anisotropy, giving rise to the ZT peak values of p-type doping in xx- and yy-directions being 0.81 and 1.99, respectively, and those of n-type doping reach ultrahigh values of 7.12 and 2.89 at 300 K, which are obviously higher than the reported values for p-type and n-type doping ternary compound Sn₂BiX (ZT∼ 1.70 and ∼2.45 at 300 K) (2020 Nano Energy 67 104283). This work demonstrates that 2D Ga₂I₂S₂ has high anisotropic TE conversion efficiency and can also be used as a new potential room-temperature TE material.