We developed a new dansyl phthalimide-based fluorescent chemosensor for hydrazine detection. Upon a Gabriel type-based hydrazinolysis of dansyl phthalimide (DPI) in the presence of hydrazine in a mixture of HEPES buffer (pH 7.0, 20 mM) and DMSO (1/9, v/v) at room temperature, the chemosensor produces fluorescent dansyl-NH2 with the maximum emission wavelength changed from 475 nm to 512 nm along with a color change from yellow to colorless, allowing colorimetric detection of hydrazine by the naked eye. DPI can selectively detect hydrazine over other environmentally abundant ions. Moreover, DPI coated with silica gel TLC plates could act as a visual and fluorimetric probe for hydrazine vapor at a partial pressure of 5.5 × 10-3 mm Hg over other potentially interfering volatile analytes, including hydrogen peroxide, ethylenediamine, urea, ammonium hydroxide and methylamine. DPI can also be used for the detection of hydrazine in water samples and HeLa cells without appreciable interference from other biologically abundant analytes. The limit of detection is 6.01 ppb (1.88 × 10-7 M), which is well below the accepted limit (10 ppb) for hydrazine set by the U.S. Environmental Protection Agency (EPA).