The water dimer is an ideal chemical species with which to study hydrogen bonds. Owing to the equilibrium between the monomer and oligomer structure, however, selective generation and separation of a genuine water dimer has not yet been achieved. Here, we report a synthetic strategy that leads to the successful encapsulation of one or two water molecules inside fullerene C-70. These endohedral C-70 compounds offer the opportunity to study the intrinsic properties of a single water molecule without any hydrogen bonding, as well as an isolated water dimer with a single hydrogen bond between the two molecules. The unambiguously determined off-centre position of water in H2O@C-70 by X-ray diffraction provides insights into the formation of (H2O)(2)@C-70. Subsequently, the H-1 NMR spectroscopic measurements for (H2O)(2)@C-70 confirmed the formation of a single hydrogen bond rapidly interchanging between the encapsulated water dimer. Our theoretical calculations revealed a peculiar cis-linear conformation of the dimer resulting from confinement effects inside C-70.