In the title organic–inorganic hybrid complex, poly[[[μ-3,5-bis(3-pyridyl)-1,2,4-triazole]tri-μ3-oxido-tetra-μ2-oxido-oxidodicobalt(II)dimolybdenum(VI)] monohydrate], {[Co2Mo2O8(C12H9N5)]·H2O}n, the asymmetric unit is composed of two CoIIcenters, two [MoVIO4] tetrahedral units, one neutral 3,5-bis(3-pyridyl)-1,2,4-triazole (BPT) ligand and one solvent water molecule. The cobalt centers both exhibit octahedral [CoO5N] coordination environments. Four CoIIand four MoVIcenters are linked by μ2-oxide and/or μ3-oxide bridges to give an unprecedented bimetallic octanuclear [Co4Mo4O22N4] cluster, which can be regarded as the first example of a metal-substituted octamolybdate and exhibits a structure different from those of the eight octamolybdate isomers reported to date. The bimetallic oxide clusters are linked to each other through corner-sharing to give two-dimensional inorganic layers, which are further bridged bytrans-BPT ligands to generate a three-dimensional organic–inorganic hybrid architecture with six-connected distorted α-Po topology.