Single crystals of BeC4O4 · 3 H2O were prepared from aqueous solution. The X-ray structure determination revealed a new coordination mode of the squarate dianion. C4O4 2- is bound to Be2+ by only one of its oxygen atoms. Three water molecules join the Be coordination sphere to form a slightly distorted tetrahedron together with the Osquarate atom. Be-Osquarate bond lengths are negligibly shorter than Be—OH2 distances. Strong hydrogen bonds occur between water molecules and all Οsquarate atoms not bound to Be2+. These Osquarate atoms act as proton acceptors in two hydrogen bonds, each of almost equal strength. One intramolecular hydrogen bond occurs within the BeC4O4.3 H2O moieties which are connected to each other by intermolecular hydrogen bonds. C4O4 2- does not deviate markedly from planarity. Difference electron density maps show evidence for bent bonds in the C4 ring system of C4O4 2-.