The title compound, [Cu4(C2O4)2(N3)4(C5H9N3)2]·0.9H2O, contains a tetranuclear Cu(II)-based molecule composed of two oxalate-bridged Cu(II) dimers linked through end-on azide ions and related by an inversion center. The tetranuclear unit contains two crystallographically independent Cu(II) ions. One Cu(II) ion coordinates to two N atoms of a histamine mol-ecule, two O atoms of a bridging oxalate ligand, and an N atom of an end-on bridging azide ligand, leading to an elongated square-pyramidal coordination geometry in which the azide ion occupies the axial position. The other Cu(II) ion, which has a square-planar coordination geometry, is coordinated by two O atoms of a bridging oxalate ligand and two N atoms of two different azide ligands, one which is bridging. In the crystal, a two-dimensional network parallel to (010) is formed by N-H⋯N and N-H⋯O hydrogen bonds. A partially occupied solvent water mol-ecule refined to an occupancy of 0.447 (5). Two of the azide ligands were refined as disordered over two sets of sites with refined occupancies in the ratios 0.517 (8):0.483 (8) and 0.553 (5):0.447 (5).