The specific sensing of an exact G-quadruplex structure by small molecules has never been reported. A fluorescent sensor based on the photoinduced electron transfer (PeT) mechanism provides possibilities for such specific, one-to-one recognition, indicated by fluorescence. We have rationally developed a PeT fluorescent sensor IZFL-2 by linking triarylimidazole and fluorescein moieties. IZFL-2 is a distinctive, smart sensor whose fluorescence is tunable by its molecular conformations. We then applied IZFL-2 to sensing G-quadruplexes and found that it could exactly distinguish the wild-type c-MYC G-quadruplex from other types of G-quadruplexes, as shown by the activation of its fluorescence. To understand this behavior, we performed various experiments, including fluorescence assays, absorption assays, and multiscale molecular dynamics simulations, to thoroughly investigate the optimal binding mode of IZFL-2 in the c-MYC G-quadruplex. Then, the corresponding HOMO-LUMO of IZFL-2 was analyzed, and the results demonstrated that the PeT process of IZFL-2 is suppressed only in the wild-type c-MYC G-quadruplex via specific loop interactions, which restores its fluorescence. To our knowledge, this smart molecule provides the first example of and new insights into the development of sensors specific for a particular G-quadruplex structure by utilizing intramolecular PeT-controlled fluorescence switching.