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A new class of materials, antiferromagnetic topological insulators, has been predicted to realize the bulk axion insulator with a large and in some cases quantized magneto-electric coefficient. Hexagonal MnBi₂Te₄ is a candidate material and here we report neutron scattering experiments conducted at the NIST Center for Neutron Research to understand its magnetism. We confirm A-type antiferromagnetic order (alternating FM basal planes with spins along the hexagonal axis) with T_N=24 K. Inelastic scattering reveals an excitation spectrum with a bandwidth of 4.5 meV and gap △<0.6 meV. Modelling these data using spin-wave theory, we obtain an effective spin Hamiltonian for MnBi₂Te₄ that features competing ferro- and antiferromagnetic interactions within the basal plane.