We conducted in situ angle dispersive high pressure x-ray diffraction experiments on Sr 3 Ir 2 O 7 up to 23.1 GPa at 25 K with neon as the pressure transmitting medium. Pressure induces a highly anisotropic compressional behavior seen where the tetragonal plane is compressed much faster than the perpendicular direction. By analyzing different aspects of the diffraction data, a second-order structural transition is observed at approximately 14 GPa, which is accompanied by the insulating state to nearly metallic state at 13.2 GPa observed previously (Li et al 2013 Phys. Rev. B 87 235127). Our results highlight the coupling between electronic state and lattice structure in Sr 3 Ir 2 O 7 under pressure.