The binary oxide Np2O5 has been synthesized by mild hydrothermal reaction and its crystal structure has been determined. It is monoclinic, P2/c, Z = 4, a = 8.168(2), b = 6.584(1), c = 9.313(1) A, beta = 116.09(1)degrees, V = 449.8(2) A(3). The structure contains neptunyl square and pentagonal bipyramids that are linked into sheets by edge-sharing. Cation-cation interactions occur within the sheet of polyhedra and provide linkages of the sheets into a three-dimensional structure. Np2O5 undergoes antiferromagnetic ordering at 22(3) K, the highest magnetic ordering temperature observed for any neptunyl compound, and is only the second neptunyl compound found to order antiferromagnetically.