We present herein a fluorescence-based method for the determination of both the trivalent and hexavalent forms of chromium in aqueous solutions using unmodified gold nanoparticles. The concept of the sensor was designed based on the aggregation of gold nanoparticles (Au NPs) by Cr(III), which results in a color change from red to blue and the appearance of a new secondary peak at 714 nm. The complexation of Au NPs by Cr(III) consequently leads to the quenching of the fluorescence intensity of Au NPs, which is proportional to the concentration of Cr(III). The Au NP aggregation upon the addition of Cr(III) is well correlated with the mean hydrodynamic size measurements and scanning electron microscopy images. The system was found to possess a good linear correlation between the chromium concentration and the degree of reduction of fluorescence intensity (R² = 0.989) in the range of 10⁻⁷–10⁻³ M with an excellent limit of detection of 10⁻⁷ M (5 ppb). The prospective application of the as-designed probe for environmental sensing can be highlighted because it has been found to successfully determine the chromium concentration in real water samples. Our method has the advantage of cost-effectiveness and does not use any additional fluorophores for the sensitive detection of both the forms of chromium.