A metal‐organic polymer [Ni3(μ3‐O)(PDB)3]·H2O (1) (PDB = pyridine‐3,5‐dicarboxylate), with antiferromagnetic interactions between the adjacent Ni atoms, containing trinuclear μ3‐oxo‐bridged metal units Ni3(μ3‐O) have been synthesized by hydrothermal reaction of the achiral building blocks pyridine‐3,5‐dicarboxylate (3,5‐PDB) and Ni(NO3)2·4H2O. Compound 1 shows a high symmetry three‐dimensional snowflake‐shaped (3, 9)‐connected topology structures in which μ3‐oxo mixed‐valence Ni3O(CO2)6 clusters act as nine‐connected nodes and PDB ligands act as three‐connected nodes.