It is important to prepare high-quality, ultrathin dielectric insulators on graphene for the integration of graphene field-effect transistors. Here, we present a novel and facile approach to deposit high-κ dielectrics (such as Al 2 O 3 ) by atomic layer deposition (ALD) through water physisorption. Usually it is difficult to grow Al 2 O 3 thin films on pristine graphene directly using H 2 O as precursor. The pretreatment of graphene with ALD H 2 O pulses before the deposition can enhance the Al 2 O 3 coverage on pristine graphene, but the film is not continuous yet. It is found that uniform Al 2 O 3 thin films can easily deposit on the graphene after H 2 O dipping pretreatment for ∼4–5h. It is ascribed to the fact that the physisorbed water molecules on the surface of graphene as nucleation sites react with the metal precursor of trimethylaluminum (TMA) to form Al 2 O 3 dielectric films. Raman spectra reveal that no defects are produced on graphene surface via this method. These results indicate that water functionalization is a promising pathway to achieve scaled gate dielectrics on graphene without introducing any defects.