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The crystal and magnetic structure of strontium ferrite Sr 4 Fe 4 O 11 (SrFeO 2.75 ) has been investigated using neutron powder diffraction and magnetic susceptibility measurements over the temperature range 1.5-293 K and by Mössbauer spectroscopy at room temperature. Sr 4 Fe 4 O 11 is orthorhombic (symmetry group Cmmm) over the temperature range 1.5-293 K. Its crystal structure contains two iron sites occupied by trivalent and tetravalent iron. The trivalent magnetic moments order antiferromagnetically below T N = 232(4) K and the magnetic unit cell coincides with the chemical cell. The ordered moments are parallel to the b axis and the tetravalent iron moments form a magnetically frustrated sublattice. The extrapolated value of the Fe 3+ moment is 3.55(5) µ B at 0 K. The antiferromagnetic coupling occurs by the superexchange mechanism and is mediated by oxygen ions. There are no signs of long-range magnetic order in the tetravalent sublattice down to 1.5 K.