The crystal structure of katoite hydrogarnet has been refined at 0.0001, 2.15, 4.21, 5.09, 6.00, 7.09, and 7.78 GPa from single-crystal X-ray diffraction data using a 4:1 methanol:ethanol mixture as pressure medium in a Merrill-Bassett diamond-anvil cell. Below ~5 GPa, the katoite structure has Ia3d symmetry and compresses by bond shortening rather than bond bending, in agreement with recent quantum mechanical calculations. An unconstrained third-order Birch-Murnaghan fit to the unit-cell volumes and pressures for Ia3d symmetry gave the following equation of state parameters: Vo = 1987.6(1) Å3, Ko = 58(1) GPa and K′ = 4.0(7). Above this pressure, the structure undergoes a phase transition to space group I4̅3d, a non-centric subgroup of Ia3d. In the I4̅3d structure, there are two non-equivalent (O4H4) groups. Both the Ca and Al atoms are displaced along a relative to their positions in Ia3d. It is proposed that compression of the short H-H distance between (O4H4) groups destabilizes the structure and may initiate the observed phase transition. Corroboration of this model will require accurate information on the hydrogen atom positions at pressures above 5 GPa.