The asymmetric unit of the title compound, C(8)H(10)O(8)·2H(2)O, contains one half-mol-ecule of butane-1,2,3,4-tetra-carboxylic acid and a water mol-ecule, with the complete tetra-acid generated by crystallographic inversion symmetry. Inter-molecular O-H⋯O hydrogen bonds form an extensive three-dimensional network, which consolidates the crystal packing.