The structure of the ionic title compound, (C(5)H(7)N(6))(2)[Nd(2)(C(5)O(5))(4)(H(2)O)(8)], consists of anionic dimers built around an inversion centre and is made up of an Nd(III) cation, two croconate (croco) dianions and four water molecules (plus their inversion images), with two noncoordinated symmetry-related 2,6-diamino-1H-purin-3-ium (Hdap(+)) cations providing charge balance. Each Nd(III) atom is bound to nine O atoms from four water and three croco units. The coordination polyhedron has the form of a rather regular monocapped square antiprism. The croconate anions are regular and the Hdap(+) cation presents a unique, thus far unreported, protonation state. The abundance of hydrogen-bonding donors and acceptors gives rise to a complex packing scheme consisting of dimers interlinked along the three crystallographic directions and defining anionic `cages' where the unbound Hdap(+) cations lodge, linking to the mainframe via (N-H)(Hdap)···O(water/croco) and (O-H)(water)···N(Hdap) interactions.