Investigating the coordination chemistry of H2CDA (4‐oxo‐1,4‐dihydro‐2,6‐pyridinedicarboxylic acid) with rare earth salts Ln(NO3)3 under hydrothermal conditions, structure transformation phenomenon was observed. The ligand, H2CDA charged to its position isomer, enol type structure, H3CAM (4‐hydroxypyridine‐2,6‐dicarboxylic acid). Six new lanthanide(III) coordination polymers with the formulas [Ln(CAM)(H2O)3]n [Ln = La (1), Pr, (2)] and {[Ln(CAM)(H2O)3]·H2O}n [Ln = Nd, (3), Sm, (4), Eu, (5), Y, (6)] were synthesized and characterized. The X‐ray structure analyses show two kinds of coordination structures. The complexes 1 and 2 and 3–6 are isostructural. Complexes 1 and 2 crystallize in the monoclinic C2/c space group, whereas 3–6 crystallize in the monoclinic system with space group P21/n. In the two kinds of structures, H3CAM displays two different coordination modes. The SmIII and EuIII complexes exhibit the corresponding characteristic luminescence in the visible region at an excitation of 376 nm.