The presence of water in gasoline impairs fuel combustion, corrodes storage tanks and favors microbial proliferation. Therefore, there is a need for simple and sensitive methods to analyze traces of water in gasoline. Here, we developed a method based on the fluorescence of diphenylquinoxaline-6-amine (DPQA). Firstly, DPQA fluorescence is turned off in the presence of bis-(2,4,6-trichlorophenyl) oxalate and zinc perchlorate in anhydrous ethyl acetate. This is explained by the formation of a nonfluorescent complex. Then, adding water at trace levels turns DPQA fluorescence on again, a phenomenon explained by the breakdown of the non-fluorescent complex. We use this method to quantify water in gasoline. Results show an excellent linear correlation with a 0.99 coefficient between fluorescence intensity and water content in the range 0.01-0.2 v/v% with a detection limit of 0.0025 v/ v%. Overall, our findings represent the first application of a turn-on fluorescence sensor to analyze water in gasoline.