The BiMnO3 perovskite is a very interesting multiferroic material that, once synthesized at high pressure and high temperature, survives as a metastable phase at ambient conditions. We investigated ceramic samples prepared in different conditions (temperature, pressure, and composition), and the existence of polymorphism at room temperature was clearly evidenced by electron diffraction and high-resolution electron microscopy in all the samples. A new polymorph, characterized by a different distortion of the perovskite basic cell, was found to coexist as a minor phase with the well-known C2 monoclinic form. The new polymorph, which can be described by a triclinic (pseudorhombohedral) superstructure with a = 13.62 Å, b = 13.66 Å, c = 13.66 Å, α = 110.0°, β = 108.8°, and γ = 108.8°, is mostly segregated at the grain surface. Magnetic characterizations revealed for this second form a critical temperature of 107 K …