Herein, a novel “ON-OFF” colorimetric and fluorometric chemosensor; 1N-allyl-2-(2, 5-dimethoxyphenyl)-4, 5-diphenyl-1H-imidazole (ADPPI), was constructed for sequential determination of Cu2+ and S2− ions in aqueous media. The interaction between chemosensor ADPPI and different metal cations was investigated using UV-VIS and fluorimetric spectroscopy. ADPPI showed a favorable and good interaction with Cu2+ ions producing blue colored solution peaked at 610 nm with blue fluorescence at λem. = 447 nm. The produced complex between Cu2+ ions and ADPPI can be used as a cascade probe for detection of S2− ions. The detection limits (LODs) were 1.01 nM and 1.25 μM for Cu2+ and S2− ions, respectively (the lowest between the family of colorimetric and fluorometric chemosensors). To further increase the applicability of the proposed method, Cu2+ and S2− ions concentrations were measured in environmental water samples.