Hydrogen bond (H-bond) formation in water has been a challenging task because water molecules are constant competitors. In biological systems, however, stable H-bonds are formed by shielding the H-bonding sites from the competing water molecules within hydrophobic pockets. Inspired by the nature's elaborated way, we found that even mononucleotides (G and C) can form the minimal G center dot C Watson-Crick pair in water by simply providing a synthetic cavity that efficiently shields the Watson-Crick H-bonding sites. The minimal Watson-Crick structure in water was elucidated by NMR study and firmly characterized by crystallographic analysis. The crystal structure also displays that, within the cavity, coencapsulated anions and solvents efficiently mediate the minimal G center dot C Watson-Crick pair formation. Furthermore, the competition experiments with the other nucleobases clearly revealed the evident selectivity for the G center dot C base pairing in water. These results show the fact that a H-bonded nucleobase pair was effectively induced and stabilized in the local environment of an artificial hydrophobic cavity.