The colorimetric detection of heavy metal ions based on gold nanoparticles (GNPs) with a functionalized surface layer has received much attention due to their unique size and the dependence of their optical and electronic properties on the degree of aggregation. We developed a simple, environmentally friendly detection technique which uses GNPs functionalized with Xanthoceras sorbifolia tannin (XT) as a colorimetric probe for the selective detection of Cr³⁺ based on the aggregation-induced color change in the GNPs. In the synthesis of the XT-functionalized GNPs, XT simultaneously acts as a reducing and a stabilizing agent without requiring additional reagents or treatment. The XT-stabilized GNPs obtained are almost spherical, stable and water-soluble. The XT-stabilized GNPs can be used as a high-selectivity template to chelate with Cr³⁺; this chelation induces the aggregation of the GNPs, resulting in a color change from red to gray/purple within a few seconds. The proposed colorimetric sensor has been successfully applied to the detection of Cr³⁺ in tap water and river water. Compared with the current materials and methods for Cr³⁺ detection based on GNPs, this approach opens up a novel and environmentally friendly technique for the efficient detection of Cr³⁺.