A combined experimental and theoretical study of hydrogen-rich ammonium borohydride (NH₄BH₄) subjected to pressures up to 10 GPa indicates two phase transitions, detected by synchrotron radiation powder X-ray diffraction, Raman spectroscopy and Car–Parrinello molecular dynamics calculations, at 1.5 and 3.4 GPa. The ambient pressure, face-centred cubic phase of NH₄BH₄ transforms into a highly disordered intermediate structure which then evolves upon increasing pressure into an orthorhombic, distorted CsCl structure. The structure of the latter phase was solved using ab initio computational techniques and from a Rietveld full pattern refinement of the powder X-ray diffraction data.