We developed a naphthalimide-indoline hybrid (1) as a pH-sensitive turn-on fluorescent probe. Probe 1 displays a weak fluorescence intensity in pH span of 2.5–11.0 owing to a photo-induced electron transfer (PET) from the indoline moiety to the naphthalimide. However, the PET process is suppressed under the pH of 2.5, showing a strong fluorescence signal at 430 nm. The turn-on fluorescent change of 1 is selective for the acidity (H⁺) over other anions, metal ions, redox species and it displays a good reversibility. Moreover, glass TLC plates coated with probe 1 can readily detect acid vapors at an ambient atmosphere.