The precise crystal symmetry, and hence the emergence of the electric polarization, still remains an open question in multiferroic MnO (=rare earth, Bi, Y). While previous diffraction studies have indicated the centrosymmetric space group , an atomic displacement allowing for electric polarization would require a noncentrosymmetric symmetry. Here, single-crystal neutron diffraction experiments on TbMnO provide direct evidence of a reduced crystallographic symmetry with the polar space group already above the multiferroic phase transition, indicating that a symmetric spin coupling, i.e., magnetostriction is the dominant mechanism in the commensurate magnetic phase.