A new triclinic perovskite, BiNiO₃, has been synthesized at a high pressure of 6 GPa in an oxidizing atmosphere. Structure refinements based on synchrotron X-ray powder diffraction and bond-valence calculations revealed a disproportionation of the Bi ions into Bi³⁺ and Bi⁵⁺, both being located in distorted oxygen octahedra. Because of the presence of Bi⁵⁺, the oxidation state of Ni was +2, rather than +3 as had been expected; BiNiO₃ thus showed insulating behavior with localized spins of S = 1.