AbstractHerein, a strategy is proposed to simultaneously deliver the small-molecule drug metformin and siRNA with self-assembled DNA nanostructures. The biomedical application of DNA nanostructures is highly promising but still in its infancy. DNA nanostructures as drug delivery vehicles are conventionally synthesized in a magnesium-containing buffer. We propose using an anticancer drug to assemble DNA nanostructures and deliver them with siRNA for synergistic anticancer therapy. The metformin cargo induces DNA self-assembly into well-defined, uniform nanostructures, producing a drug–DNA nanocomplex with multiple functionalities for cancer therapy. Both tile-based and DNA origami structures can be assembled with metformin. The as-prepared metformin/DNA nanocomplex showed high structural and thermal stability and enzymatic resistance in physiological settings. The metformin in the nanocomplex and the KRAS^G12C siRNA exerted a strong, synergistic antitumor effect against KRAS-mutated non-small cell lung cancer (NSCLC) both in vitro and in vivo by suppressing the RAS/AKT/mTOR and AMPK/AKT/mTOR signaling pathways. The current study suggests that the assembly of complex DNA nanomaterials with carefully chosen small molecules is key to endowing DNA nanostructures with new functionalities and subsequently expanding their applications in multidisciplinary research fields.