The new oxofluoride FeSeO3F, which is isostructural with FeTeO3F and GaTeO3F, was prepared by hydrothermal synthesis, and its structure was determined by X-ray diffraction. The magnetic properties of FeSeO3F were characterized by magnetic susceptibility and specific heat measurements, by evaluating its spin exchanges on the basis of density functional theory (DFT) calculations, and by performing a quantum Monte Carlo simulation of the magnetic susceptibility. FeSeO3F crystallizes in the monoclinic space group P2(1)/n and has one unique Se4+ ion and one unique Fe3+ ion. The building blocks of FeSeO3F are [SeO3] trigonal pyramids and cis-[FeO4F2] distorted octahedra. The cis-[FeO4F2] octahedra are condensed by sharing the O-O and F F edges alternatingly to form [FeO3F], chains, which are interconnected via the [SeO3] pyramids by corner-sharing. The magnetic susceptibility of FeSeO3F is characterized by a broad maximum at 75(2) K and a long-range antiferromagnetic order below K. The latter is observed by magnetic susceptibility and specific heat measurements. DFT calculations show that the Fe F Fe spin exchange is stronger than the Fe-O-Fe exchange, so each [FeO3F], chain is a Heisenberg antiferromagnetic chain with alternating antiferromagnetic spin exchanges. The temperature dependence of the magnetic susceptibility is well-reproduced by a quantum-Monte Carlo simulation.