The nuclear and magnetic structures of polycrystalline Sr2MnO3.5 have been determined by the Rietveld analysis of neutron powder diffraction data and electron diffraction techniques. The pure Mn3+ single-layered phase crystallizes in the primitive monoclinic space-group P21/c with lattice constants a = 6.8524(3) Å; b = 10.8131(4) Å; c = 10.8068(4) Å; β = 113.247(4)°. The oxygen defects form an ordered superstructure within the perovskite block layers consisting of interconnected MnO5 square pyramids, slightly different from those observed for the defect perovskites SrMnO2.5 and Ca2MnO3.5. Magnetic susceptibility studies show a broad transition at ∼280K, which is attributed to an overall antiferromagnetic ordering of spins, which leads to doubling of the unit cell along . The magnetic unit cell comprises ferromagnetic clusters of four corner-sharing MnO5 pyramids, which are antiferromagnetically aligned to other similar clusters within the perovskite block layers.