A phthalimide-based dye, probe 1, was developed as a novel fluorescent probe for thiol detection with excellent selectivity and high sensitivity based on the combination of photo induced electron transfer (PET) and excited state intramolecular proton transfer (ESIPT) mechanisms. The probe can detect thiols quantitatively and selectively with a large Stokes shift (161 nm) and the detection limit (S/N = 3) is as low as 0.8 nM. Furthermore, this probe was successfully used for imaging thiols in living SH-SY5Y cells.