Metal-mediated artificial DNA base pairs, consisting of two ligand-type nucleosides and a bridging metal ion, are promising building units for constructing DNA-based supermolecules. Metallo-base pairing allows the thermal stabilization and the site-specific functionalization of DNA duplexes. In this review, representative examples of the metallo-base pairs are classified according to the metal species, and their structures and properties are highlighted. Recent applications including polymerase synthesis are also overviewed to illustrate the future directions of this research field.