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The title compound, (C3H7N6)4[Ca(C7H3NO4)3]·6H2O or (tataH)4[Ca(pydc)3]·6H2O (where tata is 2,4,6-triamino-1,3,5-triazine and pydcH2 is pyridine-2,6-dicarboxylic acid), was obtained by reaction of Ca(NO3)2·4H2O with the proton-transfer compound (tataH)2(pydc) in aqueous solution. The [Ca(pydc)3]4− anion has twofold crystallographic symmetry. It is a nine-coordinate CaII complex with a distorted tricapped trigonal-prismatic coordination geometry. The structure also contains four tataH+ cations and six uncoordinated water molecules. There are extensive O—H...O, O—H...N, N—H...O, N—H...N and C—H...O hydrogen bonds in the crystal structure.