Two-dimensional materials with a competitive figure of merit zT are extremely desirable for the fabrication of thermoelectric modules. In the present work, we systematically evaluated the thermoelectricity of monolayer 2H-ZrI2 through first-principles calculations and semiclassical Boltzmann transport theory. Apart from its all-round structural stability and feasibility of experimental preparation, the semiconducting material with a moderate indirect bandgap of 1.07 eV at the HSE06 level exhibits an acceptable lattice thermal conductivity κL of 6.69 W m-1 K-1 at 900 K contributed dominantly by acoustic modes. The four-phonon scattering is found to exert a minor effect on the κL of the monolayer. Moreover, a quite large power factor of up to 147.89 mW m-1 K-2 can be achieved due to the conduction band valley degeneracy near the Fermi level. These transport coefficients eventually give rise to a significant optimal n-type zT value as high as 3.57 at 900 K at a certain experimentally achievable electron doping concentration. Our study demonstrates the potential application of monolayer 2H-ZrI2 in high-temperature thermoelectric devices.