AbstractSelective dissolution of hafnium‐peroxo‐sulfate films in aqueous tetramethylammonium hydroxide enables extreme UV lithographic patterning of sub‐10 nm HfO2 structures. Hafnium speciation under these basic conditions (pH>10), however, is unknown, as studies of hafnium aqueous chemistry have been limited to acid. Here, we report synthesis, crystal growth, and structural characterization of the first polynuclear hydroxo hafnium cluster isolated from base, [TMA]6[Hf6(μ‐O2)6(μ‐OH)6(OH)12]⋅38 H2O. The solution behavior of the cluster, including supramolecular assembly via hydrogen bonding is detailed via small‐angle X‐ray scattering (SAXS) and electrospray ionization mass spectrometry (ESI‐MS). The study opens a new chapter in the aqueous chemistry of hafnium, exemplifying the concept of amphoteric clusters and informing a critical process in single‐digit‐nm lithography.