The high pressure behaviour of Li₃Na₃In₂F₁₂ garnet (Ia¯3d, Z = 8) is studied up to 9.2 GPa at room temperature in diamond anvil cells using xray diffraction. Its equation of state to 9.2 GPa and the pressure dependences of the structural parameters to 4.07 GPa are determined from synchrotron angle-dispersive powder and laboratory single-crystal data, respectively. No indication of any structural phase transition in this material has been found up to 9.2 GPa. The fitting of the Murnaghan equation of state yields B0 = 36.2(5) GPa, B₀ = 5.38(18), and V₀ = 2051.76(0.69) °A ³. The compressibility mechanism of Li₃Na₃In₂F₁₂ is attributed to the substantial bending of the In-F-Li angles linking the InF₆ octahedra and LiF₄ tetrahedra. The most compressible polyhedral units are the NaF₈ triangulated dodecahedra. These results are discussed in relation to previous high pressure photoluminescence measurements and compared with the high pressure behaviour of silicate garnets.