A coordination polymer formulated as [Mn(5-pncH₂)₂H₂O] (1, 5-pncH₃ = 5-phosphono-1- naphthalenecarboxylic acid), was hydrothermally synthesized and magnetically characterized. Compound 1 features a three-dimensional structure in which the inorganic chains of [Mn(O-P-O)]_n are cross-linked by the organic groups of phosphonate ligands. Magnetic measurements of 1 indicate that dominant antiferromagnetic interactions are mediated between the Mn^II centers.