MXene monolayers have received increasing attention due to their unique properties, particularly their high conductivity, which shows great potential in thermoelectric materials. In this paper, we present a theoretical study of the thermoelectric properties of X3N2O2 (X = Hf, Zr) MXene monolayers, taking electron-phonon coupling into consideration. Owing to their similar geometrical structures, electronic band structures, and phonon dispersions, X3N2O2 MXene monolayers exhibit homogeneous electron and phonon transport properties. The conduction band shows multi-valley characteristics which leads to better n-type electron transport properties than p-type ones. The maximum values of the n-type power factor can reach 32 μW cm−1 K−2 for the Hf3N2O2 monolayer and 23 μW cm−1 K−2 for the Zr3N2O2 monolayer. In terms of phonon transport, the lattice thermal conductivity for the Zr3N2O2 monolayer is higher than that for the Hf3N2O2 monolayer, due to larger phonon group velocity. Our results show that the Hf3N2O2 monolayer is more suitable for thermoelectric materials than the Zr3N2O2 monolayer, with optimal n-type thermoelectric figure of merit (ZT) values of 0.36 and 0.15 at 700 K, respectively. These findings may be useful for the development of wearable thermoelectric devices and sensor applications based on X3N2O2 MXene monolayers.