A unique 3D fourfold interpenetrated metal-organic framework, [Co(L)(H2O)2]·H2O (1), has been synthesized by the solvothermal reaction of H2L with Co(NO3)2·6H2O (H2L = 5-(isonicotinamido)isophthalic acid). Compound 1 crystallizes in the monoclinic space group P21/c, with the cell parameters: a = 81301(8), b = 107711(11), c = 167697(16) Å, β = 92.656(2)◦ ,V = 14669(3) Å3, R1 = 0.0325 and wR2 = 0.0833. Its framework has (10,3)-b topology, where the cobalt atoms are alternately bridged by the pyridyl and the carboxylate groups of the L2− ligands into a three-dimensional network. Compound 1 displays antiferromagnetic interactions. Above 40 K, χm−1 obeys the CurieWeiss law with C = 3.28 emu K mol−1 andΘ = −0.66 K.