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We report the magnetic properties and magnetic structure determination for a linear-chain antiferromagnet, MnBi₂Se₄. The crystal structure of this material contains chains of edge-sharing MnSe₆ octahedra separated by Bi atoms. The magnetic behavior is dominated by intrachain antiferromagnetic (AFM) interactions, as demonstrated by the negative Weiss constant of −74 K obtained by the Curie–Weiss fit of the paramagnetic susceptibility measured along the easy-axis magnetization direction. The relative shift of adjacent chains by one-half of the chain period causes spin frustration due to interchain AFM coupling, which leads to AFM ordering at <i>T</i>_N = 15 K. Neutron diffraction studies reveal that the AFM ordered state exhibits an incommensurate helimagnetic structure with the propagation vector <i>k</i> = (0, 0.356, 0). The Mn moments are arranged perpendicular to the chain propagation direction (the crystallographic <i>b</i> axis), and the turn angle around the helix is 128°. The magnetic properties of MnBi₂Se₄ are discussed in comparison to other linear-chain antiferromagnets based on ternary mixed-metal halides and chalcogenides.