A phthalimide probe (P1) possessing a hydroxylamine group on the benzene ring has been prepared for fluorescence sensing of copper ions. The detection is based on the reaction between hydroxylamine and copper ions, resulting in two fluorescent products through hydroxyl rearrangement and dehydroxylation reactions. P1 shows a specific and sensitive fluorescence response towards copper ions with a limit of detection (LOD) of 1.11 nM ( N = 3). The copper impurities from the industrial sources of the "click" ligand (tris(benzyltriazolylmethyl)amine (TBTA)) have been successfully examined using P1. This is the first case to utilize the reaction between hydroxylamine and copper ions. More importantly, the copper mediated hydroxyl rearrangement reaction opens a way to prepare a new sort of excited state intramolecular proton transfer (ESIPT) dye with ultra-small size and bright green fluorescence under physiological conditions.