ilver azide (AgN ₃) was compressed up to 51.3 GPa. The results reveal a reversible second-order orthorhombic-to-tetragonal phase transformation starting from ambient pressure and completing at 2.7 GPa. The phase transition is accompanied by a proximity of cell parameters aand b, a 3° rotation of azide anions, and a change of coordination number from 4-4 (four short, four long) to eight fold. The crystal structure of the high pressure phase is determined to be inI4/mcm space group, with Ag at 4a, N₁ at 4d, and N₂ at 8h Wyckoff positions. Both of the two phases have anisotropic compressibility: the orthorhombic phase exhibits an anomalous expansion under compression along a-axis and is more compressive along b-axis than c-axis; the tetragonal phase is more compressive along the interlayer direction than the intralayer directions. The bulk moduli of the orthorhombic and tetragonal phases are determined to be K_OT = 39 ± 5 GPa with K_OT’ = 10 ± 7 and K_OT = 57 ± 2 GPa with K_OT’ = 6.6 ± 0.2, respectively.