Maleic hydrazide, a unique example of polymorphic structures, was analyzed at pressure up to 5 GPa using in situ high-pressure Raman scattering and synchrotron X-ray diffraction techniques. Changes in the Raman spectra at 2 GPa indicate that a pressure-induced phase transition is occurring. The transition was further analyzed with angle dispersive X-ray diffraction, which demonstrated that maleic hydrazide underwent a polymorphic transformation from the form III to the form II. Moreover, the observed transformation was partially reversible when the system was brought back to ambient pressure. This work suggests that the high-pressure polymorphic transformation is caused by changes in the hydrogen-bonded ribbons which lead to supramolecular rearrangements in the crystal structure.