AbstractRecently there have been notable synthetic successes in supramolecular polymerization. By contrast, it has long been known that DNA can undergo supramolecular polymerization (concatemerization). Concatemerization is a step‐like polymerization and consequently suffers from broad molecular weight distributions and generally undesirable cyclization reactions. Here we demonstrate that another supramolecular polymerization of DNA, hybridization chain reaction (HCR), is in fact a living polymerization. After consumption of initial monomer, the polymerization can be continued with further addition of monomer, and the molecular weight can be varied by the ratio of monomer to initiator. In contrast to concatemerization, HCR produces polymers with narrow dispersity while avoiding cyclization. Identification of the living character of this supramolecular polymerization presents new opportunities in structural DNA nanotechnology and molecular biology.