Nitrite (NO2−), an important intermediate in the biological nitrogen cycle, can be considered as a life-threatening marker since it leads to the production of carcinogenic nitrosamines when it reacts with secondary amines in the digestive system, besides being a methemoglobinemia risk factor in the children. Herein, we reported a fluorescence detection method for rapid, selective, and sensitive detection of NO2− in water samples. The method is based on the modification of α-MnO2 nanorods with fluorescein dye (FLR@ α-MnO2 NR) where the former acts as a quencher and the latter acts as a fluorescence reporter. After the addition of NO2− to the FLR@ α-MnO2 NR system, it reduces α-MnO2 NR to soluble Mn2+ and liberates FLR, restoring the fluorescence of FLR (Turn On). The nanoprobe, with λex/λem at 490/518 nm, has a linear range of 0.83–67.0 µM with a limit of detection (LOD) of 0.27 µM, for analysis of NO2−. The proposed method was successfully applied to the determination of NO2− in natural water samples.