We have extensively explored the high-pressure structures of transition-metal diborides (TMB2, TM = Sc, Ti, Y, and Zr) stabilized with the AlB2-type structure at ambient pressure by using first-principles structural prediction. We find two novel high-pressure structures: (i) a monoclinic structure (C2/m, Z = 4) for ScB2 and YB2 stable above 208 and 163 GPa, respectively; and (ii) a tetragonal alpha-ThSi2-type phase (I4(1)/amd, Z = 4) for TiB2 stable above 215 GPa. Our calculations show that the electron transfer from transition-metals TM to B under pressure might be the main cause for the structural phase transitions. Further phonon and hardness calculations suggest that alpha-ThSi2 phase of TiB2 is quenchable to ambient pressure and possesses excellent mechanical property with a Vickers hardness of 29.8 GPa. Interestingly, ZrB2 is quite stable and persists on the ambient-pressure AlB2-type structure up to at least 300 GPa. We attribute the strong covalent hybridization between the transition-metal Zr and B to this ultrastability.