The search for new two-dimensional (2D) harvesting materials that directly convert (waste) heat into electricity has received increasing attention.In this work,thermoelectric (TE) properties of monolayer square-Au2S are accurately pre-dicted using a parameter-free ab initio Boltzmann transport formalism with fully considering the spin-orbit coupling (SOC),electron-phonon interactions (EPIs),and phonon-phonon scattering.It is found that the square-Au2S monolayer is a promising room-temperature TE material with an n-type (p-type) figure of merit ZT =2.2 (1.5) and an unexpected high n-type ZT =3.8 can be obtained at 600 K.The excellent TE performance of monolayer square-Au2S can be attributed to the ultralow lattice thermal conductivity originating from the strong anharmonic phonon scattering and high power factor due to the highly dispersive band edges around the Fermi level.Additionally,our analyses demonstrate that the explicit treatments of EPIs and SOC are highly important in predicting the TE properties of monolayer square-Au2S.The present findings will stimulate further the experimental fabrication of monolayer square-Au2S-based TE materials and offer an in-depth insight into the effect of SOC and EPIs on TE transport properties.