Nanoscale confinement provides an ideal platform to rouse some exceptional reactions which cannot happen in the open space. Intuitively, H2 and H2O cannot react. Herein, through utilizing small-sized fullerenes (C24, C26, C28, and C30) as nanoreactors, we demonstrate that a hyperhydrogenated water species, H4O, can be easily formed using H2 and H2O under ambient conditions by ab initio molecular dynamics simulations. The H4O molecule rotates freely in the cavity of the cages and maintains its structure during the simulations. Further theoretical analysis indicates that H4O in the fullerene possesses high stability thermodynamically and chemically, which can be rationalized by the electron transfer between H4O and the fullerene. This work highlights the possibility of utilizing fullerene as a nanoreactor to provide confinement constraints for unexpected chemistry.