A highly sensitive and selective off-on bis-BODIPY fluorescent sensor, BODIPY-NN, could be used to detect Cu²⁺ ions in aqueous solutions and to image intracellular Cu²⁺ ions in living cells. Upon addition of Cu²⁺ ions, BODIPY-NN showed a turn-on fluorescence response with 4-fold enhancement at 529 nm, indicating the inhibition of photo-induced electron transfer (PET) from the phenylenediamine-triazole unit to the BODIPY skeleton. This was triggered by oxidative C-N cyclization of phenylenediamine-triazole to in situ form a benzimidazole-triazole receptor providing an appropriate coordination environment that enhances metal ion binding efficiency. A mechanistic study revealed that Cu²⁺ acts as both a catalyst for oxidative C-N cyclization and a coordination center. The detection limit was determined to be 85 nM. Importantly, BODIPY-NN has been successfully utilized for fluorescence imaging of Cu²⁺ ions in MCF-7 cells and showed no toxicity toward MCF-7 cells. The non-cytotoxic nature and high sensitivity of BODIPY-NN pave a way for its use in in vivo studies as a promising fluorescent probe for high-performance sensing and bioimaging.