The pressure evolution of the local structure and dynamics of polar nanoregions in PbSc0.5Nb0.5O3 relaxor ferroelectric is analysed by Raman spectroscopy. The pressure dependence of phonon modes up to 10 GPa reveals three characteristic pressures related to changes in the local structure: near 2 GPa, at which ferroic ordering in the Pb system occurs; near 4 GPa, at which significant structural transformations, involving decoupling of Pb and B-cations in polar nanoclusters and suppression of the B-cation off-centring take place; and near 6 GPa, at which the system reaches a saturation state. The structural transformations observed in PbSc0.5Nb0.5O3 are compared to those in PbSc0.5Ta0.5O3 and other Pb-based perovskite-type relaxor ferroelectrics.