The valence state change of BiNiO3 perovskite under pressure has been investigated by a powder neutron diffraction study and electron ic-state calculations. At ambient pressure, BiNiO3 has the unusual charge distribution Bi0.53+Bi0.55+Ni2+O3 with ordering of Bi3+ and Bi5+ charges on the A sites of a highly distorted perovskite structure. High-pressure neutron diffraction measurements and bond valence sum calculations show that the pressure-induced melting of the charge disproportionated state leads to a simultaneous charge transfer from Ni to Bi, so that the high-pressure phase is metallic Bi3+Ni3+O3. This unprecedented charge transfer between A and B site cations coupled to electronic instabilities at both sites leads to a variety of ground states, and it is predicted that a Ni-charge disproportionated state should also be observable.