Luminescent materials have many interesting applications, but it remains difficult to control the luminescence of organic materials and in particular to retain the same luminescence in solution and in the solid state, a property of interest for various imaging applications. In the present work, the fluorescent properties of the salt of 2,6-diaminopyridinium with dihydrogen phosphate have been explored. As a result of proton transfer from phosphoric acid to the pyridine nitrogen and the stabilizing effect of the two primary amines at the positions ortho to the pyridine nitrogen, the band gap between the HOMO and the LUMO is considerably diminished in comparison with that in 2,6-diaminopyridine. This is confirmed by a red shift in its absorption spectrum. Because protonation is retained in aqueous solution, the dissolved 2,6-diaminopyridinium dihydrogen phosphate salt retains a similar fluorescent spectrum as in the solid state. The crystals have been studied by single-crystal X-ray diffraction; FTIR, Raman, UV-vis-NIR, and luminescence spectroscopy; HOMO-LUMO calculations using DFT; and thermal analysis. The compound provides an example of a supramolecular motif that controls the crystal structure and the luminescence properties. In addition, the crystal exhibits negligible thermal expansion over a temperature interval of 150 °C. In short, 2,6-diaminopyridinium dihydrogen phosphate is an interesting compound for the design of luminescent devices.