We synthesize a new member of the AMnO(3) perovskite manganite family (where A is a trivalent cation)thallium manganite, TlMnO3under high-pressure (6 GPa) and high-temperature (1500 K) conditions and show that the structural and magnetic properties are distinct from those of all other AMnO(3) manganites. The crystal structure of TlMnO3 is solved and refined using single-crystal X-ray diffraction data. We obtain a triclinically distorted structure with space group P (1) over bar (No. 2), Z = 4, and lattice parameters a = 5.4248(2) angstrom, b = 7.9403(2) angstrom, c = 5.28650(10) angstrom, alpha = 87.8200(10)degrees, beta = 86.9440(10)degrees, and gamma = 89.3130(10)degrees at 293 K. There are four crystallographic Mn sites in TlMnO3 forming two groups based on the degree of their JahnTeller distortions. Physical properties of insulating TlMnO3 are investigated with Mossbauer spectroscopy and resistivity, specific heat, and magnetization measurements. The orbital ordering, which persists to the decomposition temperature of 820 K, suggests A-type antiferromagnetic ordering with the ferromagnetic planes along the [-101] direction, consistent with the measured collinear antiferromagnetism below the Neel temperature of 92 K. Hybrid density functional calculations are consistent with the experimentally identified structure, insulating ground state, and suggested magnetism, and show that the low symmetry originates from the strongly JahnTeller distorted Mn3+ ions combined with the strong covalency of the Tl3+-O bonds.