A chromogenic and “off–on” fluorogenic chemodosimeter (L) based on a naphthalene–rhodamine B derivative was designed, synthesized and characterized for the selective and sensitive detection of Au³⁺ ions in mixed acetonitrile aqueous medium in the presence of other competitive analytes. Probe L displayed a 696-fold “turn-on” fluorescence signal enhancement toward Au³⁺ ions with the detection limit of Au³⁺ of about 1.51 μM. In the presence of Au³⁺ ions, probe L also exhibited a visible color change from colorless to pink that could be easily identified by the naked eye. This colorimetric and fluorometric changes were mainly due to the Au³⁺ ion-triggered imine bond (CN) hydrolysis sensing mechanism. Moreover, theoretical calculations were performed to explain the experimental results. In addition to these features, the “off–on” probe was successfully used to monitor intracellular Au³⁺ ions in living MC3T3 cells through fluorescence imaging.