The syntheses, structural characterization, and catalytic properties of two different nickel-containing polyoxometalates (POMs) are presented. The dinickel-containing sandwich-type POM [Ni-2(P2W15O56)(2)](20) (Ni-2) exhibits an unusual aaaa geometry. The trinickel-containing WellsDawson POM [Ni-3(OH)(3)(H2O)(3)P2W16O59](9) (Ni-3) shows a unique structure where the [alpha-P2W15O56](12) ligand is capped by a triangular Ni3O13 unit and a WO6 octahedron. Ni-3 shows a high catalytic activity for visible-light-driven hydrogen evolution, while the activity for Ni-2 is minimal. An analysis of the structures of multinickel-containing POMs and their hydrogen evolution activity is given.