Resonant x-ray diffraction (RXD) measurements at the Tb L₃ and the M₅ edges were carried out on a single crystal of a rare-earth tetraboride, TbB₄, which exhibits antiferromagnetic (AFM) order breaking both space-inversion and time-reversal symmetries. Considering its crystallographic and magnetic symmetries, both the anisotropic tensor of susceptibility (ATS) scattering and the magnetic scattering contribute to space-group-forbidden odd00 reflections under the resonant conditions in the AFM phase. We found that the RXD intensity of odd00 reflections depends on the helicity of the circularly polarized incident x rays, which is reasonably explained in terms of the interference effect between the ATS and the magnetic scatterings. Furthermore, the circular dichroic RXD showed a sample-position dependence, which reflects spatial distributions of AFM domains in TbB₄. Our study reveals that RXD using circularly polarized x rays is a powerful technique to probe not only the magnitude but also the sign of AFM order parameters and to spatially resolve AFM domains in materials breaking both space-inversion and time-reversal symmetries.