Tris(hydroxymethyl)aminomethane (Tris) was successfully grafted onto the surface of a Ni-6-substituted polyoxotungstate formed in situ to further generate a three-connected polyoxometalate building block. The cooperative assembly of Tris functionalized three-connected building blocks and rigid 1,3,5-benzenetricarboxylate gave rise to an unprecedented cubic polyoxometalate organic molecular cage with high thermal and hydrothermal stability.