DNA molecular glue is a small synthetic ligand that can adhere two single-stranded DNAs that do not spontaneously hybridize with each other. For reversible control of DNA hybridization by an external light stimulus, we have developed a photoswitchable molecular glue for DNA. The photoswitchable molecular glue, NCDA, consists of two guanine-recognizing naphthyridine moieties connected with a photochromic azobenzene unit. Azobenzene undergoes a reversible cis/trans isomerization by photoirradiation, which changes the relative orientations and positions of the naphthyridine moieties, resulting into photoswitching of NCDA binding to the DNA containing GG-mismatch. NCDA in the cis configuration binds to a GG-mismatch sequence and induces the formation of the DNA duplex. Using the photoswitchable binding property of NCDA, the hybridization event of two natural unmodified DNAs can be reversibly controlled by an external light stimulus.