Integrating graphene into device architectures requires interfacing graphene with dielectric materials(1-3). However, the dewetting and thermal instability of dielectric layers on top of graphene makes fabricating continuous graphene/dielectric interfaces challenging(4-9). Here, we show that yttria (Y2O3)-a high-kappa dielectric-can form a complete monolayer on platinum-supported graphene. The monolayer interacts weakly with graphene, but is stable to high temperatures. Scanning tunnelling microscopy reveals that the yttria layer exhibits a two-dimensional hexagonal lattice rotated by 30 degrees relative to the hexagonal graphene lattice. X-ray photoemission spectroscopy measurements indicate a shift of the Fermi level in graphene on yttria deposition, which suggests that dielectric layers could be used for charge doping of metal-supported graphene.