4-Oxime-1,8-naphthalimide was reported as a novel bioorthogonal turn-on probe based on 1,3-dipolar cycloaddition reactions between in situ generated nitrile oxide and alkenes/alkynes. The resulting isoxazole products displayed dramatically strong fluorescence enhancement upon photoirradiation through isoxazole–oxazole photoisomerization. This new methodology was successfully applied for in situ fluorogenic protein labeling.