The salts hexa­aqua­magnesium furan-2,5-dicarboxyl­ate, [Mg(H₂O)₆](C₆H₂O₅), (I), and hexa­aqua­nickel furan-2,5-di­carboxyl­ate, [Ni(H₂O)₆](C₆H₂O₅), (II), provide the first crystallographic characterization of the furan-2,5-dicarboxyl­ate dianion. Both structures exhibit extensive three-dimensional hydrogen-bonding networks between the octa­hedral coordinated hexa­aquametal(II) ions and the dicarboxyl­ate anions. Although the two structures are not isomorphous, they contain essentially identical two-dimensional slabs. The distinction between the structures is that these slabs are related by translation in (II), whereas adjacent slabs in (I) are reflected relative to each other by the action of a glide plane. The reflection occurs so that the local contacts between slabs are not changed, and thus the hydrogen-bond networks are identical except for the orientation of the water mol­ecules at the inter­face between slabs.