Due to its potential applications as a high-energy density material, the high-pressure polymorphs of ammonium azide (AA) have received much attention recently. However, the crystal structure of phase II (AA-II), stable above 3.0 GPa, has remained elusive until now. By combining X-ray diffraction and Raman experiments with first-principles calculations, we determine that AA-II is a hydrogen (H)-bonded structure of ammonium and azide ions with monoclinic symmetry, space group P2/c. The latter is comprised of alternating molecular layers of ammonium and azide ions and mostly differs from AA-I by its denser packing of molecular planes while preserving the hydrogen bond network. First-principles calculations show that phase II has the lowest enthalpy among all other considered structures from 4.9 to 102.6 GPa, pushing the phase transitions to the previously predicted hydronitrogen solids to higher pressures. Raman data to 85.0 GPa at room temperature confirm the absence of phase transition and agree very well with the pressure evolution of the Raman modes of AA-II predicted by our calculations.