X‐ray circular dichroism, arising from the contrast in X‐ray absorption between opposite photon helicities, serves as a spectroscopic tool to measure the magnetization of ferromagnetic materials and identify the handedness of chiral crystals. Antiferromagnets with crystallographic chirality typically lack X‐ray magnetic circular dichroism because of time‐reversal symmetry, yet exhibit weak X‐ray natural circular dichroism. Here, the observation of giant natural circular dichroism in the Ni L3‐edge X‐ray absorption of Ni3TeO6 is reported, a polar and chiral antiferromagnet with effective time‐reversal symmetry. To unravel this intriguing phenomenon, a phenomenological model is proposed that classifies the movement of photons in a chiral crystal within the same symmetry class as that of a magnetic field. The coupling of X‐ray polarization with the induced magnetization yields giant X‐ray natural circular dichroism, revealing typical ferromagnetic behaviors allowed by the symmetry in an antiferromagnet, i.e., the altermagnetism of Ni3TeO6. The findings provide evidence for the interplay between magnetism and crystal chirality in natural optical activity. Additionally, the first example of a new class of magnetic materials exhibiting circular dichroism is established with time‐reversal symmetry. The article reveals a new type of altermagnet, in which the symmetry allows for typical ferromagnetic behaviors. It presents X‐ray circular dichroism observed in a chiral antiferromagnet with time‐reversal symmetry. This phenomenon contrasts conventional X‐ray magnetic circular dichroism. As X‐rays propagate through the material, the Berry curvature, arising from the inversion symmetry breaking, gives rise to distinct circular dichroism.