The properties of [001] symmetric tilt grain boundaries (STGBs) in magnesium oxide are investigated systematically with atomic-scale simulations. Their formation energies, atomic structures, and excess volumes are computed. STGBs are found to prefer a symmetric configuration, except for tilt angles larger than 67.4°, where the decomposition of the STGBs into ½[110] dislocations is preferred. Then, the effects of pressure are investigated from 30 up to 120 GPa, a pressure range relevant to the Earth’s lower mantle. Pressure is found to change the atomic configuration of all investigated GBs, sometimes several times as pressure increases. We also find that these changes are irreversible, the GBs retaining their high-pressure configuration even after pressure is released. Implications for the deformation of ferropericlase in the conditions of the Earth’s lower mantle are discussed.