A three-dimensional coordination polymer, [Mn-3(OH)(2)Na-2(3-cnba)6]1 (1) (3-Hcnba = 3-cyanobenzoic acid), has been synthesized by the reaction of MnCl2, NaN3, and 3-Hcnba in water. Its crystal structure was determined by single-crystal X-ray diffraction. Magnetic studies show that the complex behaves as a three-dimensional metamagnet built from effective ferrimagnetic Mn(II) chains in which spin moments are linked by interactions in an AF-F-AF (F = ferromagnetic and AF = antiferromagnetic) sequence in the triangular magnetic repeating unit.