The new aminomethylphosphonic acid H2O3P–CH2–NC5H10N–CH2–PO3H2 (H4L2) was synthesized and characterized in detail. Its reaction with Cu(NO3)2·3H2O under hydrothermal conditions led to the formation of the copper phosphonate [Cu(H2L2)], which was structurally characterized by single-crystal X-ray diffraction [space group P21/n, a = 8.4754(17), b = 16.903(3), c = 9.0787(18) Å, β = 106.58(3)°, R1 = 0.0296, wR2 = 0.0728]. In the crystal structure each Cu2+-ion is chelated by one ligand molecule forming tetragonal planar CuO2N2 polyhedra. Coordination by an oxygen atom from an adjacent second ligand molecule leads to [Cu(H2L2)]2 dimers, which contain tetragonal pyramidal CuO3N2 polyhedra. These dimers are linked by hydrogen bonds to one-dimensional chains, which are further connected by van der Waals interactions. The compound was further characterized by powder X-ray diffraction (PXRD), elemental analysis, IR spectroscopy, Raman spectroscopy, and magnetic measurements.