Novel fluorescent chemosensors (2,3-NE, 2,3-NE-Ac) containing naphthalene as fluorescent signaling moiety and urea groups as anion binding sites were designed and synthesized, and their spectral behaviors toward various anions were investigated. The chemosensor 2,3-NE was prepared by reacting 2,3-diaminonaphthalene with ethyl isocyanatoacetate and 2,3-NE-Ac was obtained by saponifying 2,3-NE with Na₂CO₃. The chemosensor 2,3-NE was shown to signal selectively the detection of fluoride ion in the fluorescence spectra in CH₃CN/DMSO (9:1, v/v) even extremely low concentration of fluoride anion. The significant spectral changes indicate that 2,3-NE can be a highly sensitive fluorescent chemosensor for fluoride anion by highly efficient quenching between the excited state of the fluorophore and fluoride ion. Job plot studies revealed that the presence of fluoride ion induces the formation of a 2:1 complex between 2,3-NE and fluoride ion. In addition, the binding mode with fluoride ion was investigated by ¹H NMR experiments. The chemosensor 2,3-NE-Ac also recognized fluoride anion, but is approximately 10-fold lower in sensitivity than 2,3-NE by the influence of the carboxylic acid groups. 2,3-NE can become a promising chemosensor for fluoride ion with high sensitivity and selectivity.