DNA sequences that undergo large changes in secondary structure upon binding of small molecules are the basis for molecular switches. Here we report a Zn2+ complex that promotes the conversion of a fully complementary DNA double helix into DNA hairpins. The conformational switch is promoted by an isolated Zn2+ complex or by free ZnCl2 and a macrocyclic ligand. The switch is selective for Zn2+ over biologically relevant transition-metal ions including Cu2+ and Fe2+. The dual ligand/DNA switch is an approach that may improve the selectivity for metal-ion-sensing applications.