A low thermal conductivity and a high power factor are essential for efficient thermoelectric materials. The lattice thermal conductivity can be reduced by reducing the dimensions of the materials, thus improving the thermoelectric performance. In this work, the electronic, carrier and phonon transport and the thermoelectric properties of ZrNX (X = Cl, Br, and I) monolayers were investigated using density functional theory and Boltzmann transport theory. The electronic and phonon transport show anisotropic properties. The thermal conductivities are 20.8, 14.6 and 12.4 W m-1 K-1 at room temperature along the y-direction for the ZrNCl, ZrNBr, and ZrNI monolayers, respectively. Combining the low lattice thermal conductivity and the high power factor results in an excellent thermoelectric performance of the ZrNX monolayers. The thermoelectric figure of merit of ZrNX monolayers can reach magnitudes of ∼0.49-3.15 by optimal hole and electron concentrations between 300 and 700 K. ZrNX monolayers with high ZT values for n- and p-type materials would thus be novel, promising candidate 2D thermoelectric materials for heat-electricity conversion.