Abstract: Up to now, “Turn-on” fluorescence sensor exhibits promising potential toward the detection of heavy metal ions, anions, drugs, organic dyes, DNA, pesticides, and other amino acids due to their simple, quick detection, and high sensitivity and selectivity. Herein, a novel fluorescence method of detecting Cr³⁺ in an aqueous solution was described based on the fluorescence resonance energy transfer between rhodamine B (RhB) and gold nanoparticles (AuNPs). The fluorescence of RhB solution could be obviously quenched (“off” state) with the presence of citrate-stabilized AuNPs. However, upon addition of Cr³⁺ to AuNPs@RhB system, the fluorescence of AuNPs was recovered owing to the strong interaction between Cr³⁺ and the specific groups on the surface of citrate-stabilized AuNPs, which will lead to the aggregation of AuNPs (“on” state). At this point, the color of the reaction solution turned to black. Under optimal conditions, the limit of detection (LOD) for Cr³⁺ was 0.95 nM (signal-to-noise ratio, S/N = 3) with a linear range of 0.164 nM to 3.270 μM. Furthermore, the proposed method exhibits excellent performances, such as rapid analysis, high sensitivity, extraordinary selectivity, easy preparation, switch-on fluorescence response, and non-time consuming.