The high pressure phases of Rb have previously been investigated to 101 GPa, above which Rb is predicted to adopt a double-hexagonal close-packed (dhcp, Pearson hP4) structure similar to that already observed in cesium at 72 GPa. Previous ab initio structure searches have indicated that the hP4 phase should become stable in rubidium at 143GPa. We present data from static compression experiments on Rb up to 264(8)GPa, showing the onset of the hP4 phase at 207(6)GPa. The V/V0 of ∼0.121 measured at 264GPa constitutes the highest compression ratio (more than eightfold) at which structural information has been obtained from a metal using x-ray diffraction methods and is second only to x-ray measurements performed on hydrogen at V/V0∼0.094 at 190GPa. At these extreme compression ratios, the compressive behavior of rubidium shifts from that of a free electron metal to that of a regular d-block metal.