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The recent discovered antiferromagnetic topological insulators in the Mn-Bi-Te family have rapidly drawn broad interest since its cleaved surface state is believed to be gapped, hosting the unprecedented axion states. However, we show by ARPES that a gapless Dirac cone at the (0001) surface of MnBi₂Te₄ exists inside the bulk band gap. Such unexpected surface state remains unchanged across the bulk Néel temperature, and is robust against severe surface degradation, indicating additional topological protection. Through symmetry analysis and ab initio calculations we consider different types of surface magneitc reconstruction as possible origins giving rise to such linear dispersion. Our results reveal that the intrinsic magnetic topological insulator hosts a rich platform to realize various topological phases, and thus pushed forward the comprehensive understanding of magnetic topological materials.