A new fluorescent probe, 6-amino-1,3-naphthalenedisulfonic acid (ANDSA), has been developed for the determination of trace nitrite in different waters. This probe is based on the selective reaction of nitrite with ANDSA in hydrochloric acid solution to form the corresponding diazonium acid in sodium hydroxide solution at room temperature. The diazonium acid produced has high fluorescence intensity at 488 nm (excitation at 367 nm), whereas ANDSA has high fluorescence intensity at 465 nm (excitation at 354 nm). The synchronous fluorescence (Δλ = 121 nm) spectrum and the first-derivative synchronous fluorescence spectrum of diazonium acid greatly overlapped with those of ANDSA. The zero-crossing method was used to measure the first-derivative value (dF/dλ) of the first-derivative spectra, in which physical separation of excess ANDSA was unnecessary. The zero-crossing point was located at 351.2 nm for ANDSA, at which dF/dλ of diazonium acid was proportional to the nitrite concentration. Therefore, dF/dλ at 351.2 nm was selected as the analytical signal. Under the optimized experimental conditions, the signal intensity was linear over a nitrite concentration range of 0.006-0.075 mg L, with a correlation coefficient better than 0.9994. The limit of detection was 2.1 μg L for the determination of nitrite by the proposed method. The relative standard deviation of the method for the determination of nitrite in real water samples was below 2.45%, and the corresponding recoveries were between 95.7% and 104.1%. The validity of the proposed method was further confirmed by comparison with the reference method with use of the t test. Graphical abstract ANDSA reacts with nitrite to form diazonium acid with higher fluorescence intensity. For ANDSA, dF/dλ was zero at 351.2 nm, whereas dF/dλ of diazonium acid at 351.2 nm was close to the maximum value.