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Monolayer silicon phosphide (SiP) and germanium phosphide (GeP) are predicted to exhibit fascinating electronic characters with highly stable structures, which indicate their potential applications in future electronic technologies. By using first-principles calculations combined with the semiclassical Boltzmann transport theory, we systematically investigate the thermoelectric properties of monolayer SiP and GeP. High anisotropy is observed in both phonon and electron transport of monolayer SiP and GeP where the thermal and electrical conductivity along the xx crystal direction are smaller than those along the yy crystal direction. The lattice thermal conductivity (room temperature) along the xx crystal direction is about 11.05 W/mK for monolayer SiP and 9.48 W/mK for monolayer GeP. However, monolayer SiP and GeP possess almost isotropic Seebeck coefficient, and the room temperature values with both n- and p-type doping approach 2.9 mV/K and 2.5 mV/K, respectively. Based on the electron relaxation time estimated from the deformation potential theory, the maximum thermoelectric figure of merit of monolayer SiP and GeP with n-type doping approach 0.76 and 0.78 at 700 K, respectively. The results presented in this work shed light upon the thermoelectric performance of monolayer SiP and GeP and foreshow their potential applications in thermoelectric devices.