*

We find a giant high-temperature optical diode effect on polar ferrimagnet FeZnMo₃O₈ using terahertz spectroscopy, where the transmitted light intensity in one direction is higher than the intensity transmitted in the opposite direction by over 100 times. Differently from all existing reports of the optical diode effect in other magnetoelectric materials where the long-range magnetic ordering is a necessary prerequisite, the effect in FeZnMo₃O₈ takes place in the paramagnetic phase with no long-range magnetic order. In FeZnMo₃O₈, the effect is observed as a resonance with a strong magnetic dipole active transition centered at 1.27 THz which is assigned as electron spin resonance between the eigenstates of the single-ion anisotropy Hamiltonian. We propose that the optical diode effect in paramagnetic FeZnMo₃O₈ is driven by single-ion terms in magnetoelectric free energy.