A pyrene compound ( 1 ) containing piperazine was synthesized and used as ratiometric fluorescent chemosensor to recognize Cr 3+ . The ratiometric fluorescent signal change of the chemosensor is on the basis of monomer–excimer conversion of 1 induced by Cr 3+ . As far as we know, it is the first ratiometric chemosensor based on pyrene that is developed for Cr 3+ . The analytical performance characteristics of the proposed Cr 3+ -sensitive chemosensor were investigated. The chemosensor can be applied to the quantification of Cr 3+ with a linear range covering from 2.0 × 10 −7 to 1.0 × 10 −5 M with the detection limit is 4 × 10 −8 M. The experiment results show that the response behavior of 1 towards Cr 3+ is pH independent in medium condition (pH 6.0–8.0). Most importantly, the fluorescence changes of the chemosensor are remarkably specific for Cr 3+ in the presence of other metal ions, which meet the selective requirements for practical application. Moreover, the response of the chemosensor toward Cr 3+ is fast (response time less than 2 min). In addition, the chemosensor has been used for determination of Cr 3+ in real sample such as river and pond water with satisfactory results.