The experimental crystal charge density of deuterated methylammonium hydrogen maleate has been determined from neutron and X-ray diffraction data collected at 122.4 K. Refinements of the neutron diffraction data showed that the very short intramolecular O-H-O hydrogen bonds found in the two independent anions are symmetric with D(H) on crystallographic mirror planes. The experimental charge density was subjected to a topological analysis. It is positive throughout the unit cell and constitutes a topological space that fulfills the Poincare-Hopf relation. The topological analysis enabled us to characterize different types of interatomic interactions in the crystal and showed that the two very short intramolecular hydrogen bonds have covalent character. Newly developed algorithms made it possible to determine the atomic basins in the crystal charge density, which were used to calculate integrated properties such as volume, charge, and the integrated Laplacian. Those of the methylammonium ion have been compared with the corresponding topological properties of the same cation in its hydrogen succinate salt. AU the properties were quantitatively transferable, a strong indication that transferability can also be applied to experimentally determined electron densities.