As a classical electron-deficient molecule with unique hydrogen bridge bonding, diborane has created considerable interest in the structural chemistry. We report here the first evidence of pressure-induced structural transformations of diborane probed by in situ Raman spectroscopy. At pressures around 4 GPa, diborane undergoes a liquid-solid phase transformation to a new high-pressure phase I with a possible structure similar to the low-temperature phase. When compressed to above 6 GPa, the spectral features, such as doubling of the lattice modes, appearance of several new internal modes, and emergence of a new ring stretch mode, indicate the structural transformation to another new high-pressure phase II. This phase has a possible extended network structure of higher hydrides of borane. At pressures above 14 GPa, diborane transforms to yet another high-pressure phase III. All of the observed pressure-induced structural transformations are completely reversible upon decompression.