Neutron diffraction and FT-IR analysis revealed that the novel oxynitrate Bi(3)Mn(4)O(12)(NO(3)) (space group P3, a = 4.9692(1) A, c = 13.1627(3) A) prepared by hydrothermal synthesis is of a new structural type including flat NO(3) layers alternating with blocks of two PbSb(2)O(6)-like layers. Mn(4+) (S = (3)/(2)) forms a regular honeycomb lattice, and magnetic susceptibility data indicated two-dimensional magnetism. Despite its Weiss constant of -257 K, no long-range ordering was observed down to 0.4 K because of the magnetic frustration due to the competition between the nearest and the next-nearest antiferromagnetic interactions.