An efficient hydrazine sensor was prepared based on the pyridomethene–BF₂ complex. It exhibits a high sensitivity in aqueous solutions despite the presence of a wide variety of anions and cations. It can also be fabricated into a solid state sensor for detection of hydrazine vapor. The fluorescence response of this sensor exhibits a ratiometric character at two different wavelengths, therefore significantly enhances the reliability of measurements. The sensing mechanism is well rationalized with the aid of DFT calculations.