A novel bifacial ligand-bearing nucleobase, 5-hydroxyuracil (U-OH), which forms both a hydrogen-bonded base pair (U-OH-A) and a metal-mediated base pair (U-OH-M-U-OH) has been developed. The U-OH-M-U-OH base pairs were quantitatively formed in the presence of lanthanide ions such as Gd-III when U-OH-U-OH pairs were consecutively incorporated into DNA duplexes. This result established metal-assisted duplex stabilization as well as DNA-templated assembly of lanthanide ions. Notably, a duplex possessing U-OH-A base pairs was destabilized by addition of Gd-III ions. This observation suggests that the hybridization behaviors of the U-OH-containing DNA strands are altered by metal complexation. Thus, the U-OH nucleobase with a bifacial base-pairing property holds great promise as a component for metal-responsive DNA materials.