Heavy rare earth metal terbium has been studied at synchrotron sources using both angular and energy dispersive x-ray diffraction to a maximum pressure of 155 GPa (volume fraction V/ V0 =0.39). The complete regular trivalent rare earth structural sequence, hcp→α-Sm→double hcp→fcc→ distorted fcc (hR24), is observed at low pressures except for the undistorted fcc. At 51±2 GPa, terbium undergoes a 5% volume collapse transitioning to C2/m monoclinic. The open, low symmetry structure is reminiscent of the 4f delocalization phenomenon in the light rare earth metals, particularly the α″ phase of cerium. © 2007 The American Physical Society.