We synthesized two high-pressure polymorphs PbNiO3 with different structures, a perovskite-type and a LiNbO3-type structure, and investigated their formation behavior, detailed structure, structural transformation, thermal stability, valence state of cations, and magnetic and electronic properties. A perovskite-type PbNiO3 synthesized at 800 degrees C under a pressure of 3 GPa crystallizes as an orthorhombic GdFeO3-type structure with a space group Pnma. The reaction under high pressure was monitored by an in situ energy dispersive X-ray diffraction experiment, which revealed that a perovskit-type phase was formed even at 400 degrees C under 3 GPa. The obtained perovskite-type phase irreversibly transforms to a LiNbO3-type phase with an acentric space group R3c by heat treatment at ambient pressure. The Rietveld structural refinement using synchrotron X-ray diffraction data and the XPS measurement for both the perovskite- and the LiNbO3-tyze phases reveal that both phases possess the valence state of Pb4+Ni2+O3. Perovskite-type PbNiO3 is the first example of the Pb4+M2+O3 series, and the first example of the perovskite containing a tetravalent A-site cation without lone pair electrons. The magnetic susceptibility measurement shows that the perovskite- and LiNbO3-type PbNiO3 undergo antiferromagnetic transition at 225 and 205 K, respectively. Both the perovskite- and LiNbO3-type phases exhibit semiconducting behavior.