In this study, by utilizing the first-principles calculation coupled with the Boltzmann transport theory, we comprehensively study the thermoelectric (TE) properties of the Sb2C monolayer. The calculated results show that the Sb2C monolayer owns an inherent ultra-low lattice thermal conductivity of 0.88 W m−1 K−1 at 300 K, which originates from small phonon group velocities, large Grüneisen parameters, and short phonon lifetimes. The Sb2C monolayer also exhibits excellent electrical transport properties mainly due to the degeneration of the bottom conduction bands, which increases the Seebeck coefficient of the n-type doped samples and thus yields a larger power factor. Based on the extremely low lattice thermal conductivity and superior electrical transport performance, a large ZT value of 2.71 for the n-type doped Sb2C monolayer at 700 K is obtained. Our results quantify Sb2C monolayers as promising candidates for building outstanding thermoelectric devices.