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Colossal magnetoresistance (CMR) is a sudden drop in the electrical resistance in response to an external magnetic field – a desirable effect for magnetic sensing and recording technologies. The established paradigm of CMR is based on the manganese oxide materials (perovskite manganates) where a mixed valence of Mn³⁺/Mn⁴⁺ leads to a ferromagnetic (FM) double-exchange (DE) interaction and a dynamical Jahn-Teller (JT) distortion, which cooperatively induce CMR. Prior efforts to find different materials or mechanisms for CMR resulted in a much smaller effect. Here we show an enormous CMR at low temperatures in EuCd₂P₂ without manganese, oxygen, mixed valence, or cubic perovskite structure. EuCd₂P₂ has a layered trigonal lattice and exhibits A-type antiferromagnetic ordering at 11 K. The magnitude of CMR (10⁴ percent) in as-grown crystals of EuCd₂P₂ rivals the magnitude in optimized thin films of manganates. Our magnetization, transport, and synchrotron X-ray data suggest that strong magnetic fluctuations are responsible for this phenomenon. The realization of CMR at low temperatures without heterovalency leads to a new regime for materials and technologies related to antiferromagnetic spintronics.