A novel supramolecular nanostructure formed by the coadsorption of the complementary nucleobases guanine (G) and uracil (U) at the liquid (1-octanol solvent)/solid (graphite) interface is revealed by scanning tunneling microscopy (STM). The GU supramolecular structure is distinctly different from the structures observed by STM when the individual nucleobases (NB) are adsorbed on graphite in the control experiments. Using a systematic methodology and ab initio density functional theory (DFT), an atomistic structural model is proposed for the supramolecular coadsorbed GU structure, which consists of a periodic repetition of cyclic units based on the strongest GU base pairing.