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Magnon-polarons, a type of hybridized excitations between magnons and phonons, were first reported in yttrium iron garnet as anomalies in the spin Seebeck effect responses. In antiferromagnetic insulators, the magnon spectrum is distinctly different due to the existence of two equivalent, antiferromagnetically coupled spin sub-lattices. Here we report an observation of antiferromagnetic (AFM) magnon-polarons in a uniaxial AFM insulator Cr₂O₃. Despite the relatively higher energy of magnon than that of the acoustic phonons, near the spin-flop transition of ~ 6 T, the left-handed magnon spectrum shifts downward to hybridize with the acoustic phonons to form AFM magnon-polarons, which can also be probed by the spin Seebeck effect. The spin Seebeck signal is founded to be enhanced due to the magnon-polarons at low temperatures.