We report a method for rapid, effective detection of both Cr(III) and Cr(VI) (in the form of Cr³⁺ and Cr₂O₇ ²⁻, the main species of chromium in the natural environment) by making use of meso-2,3-dimercaptosuccinic acid (DMSA)-functionalized gold nanoparticles (Au NPs). The limit of detection (LOD) is 10 nM with the naked eye and the assay can be applied in detecting chromium in polluted soil from Yun-Nan Province in Southwest China. We use density functional theory to calculate the change of the Gibbs free energy (ΔG) of the interactions between the DMSA-Au NP system and various metal ions, which shows that DMSA-Au NPs have high specificity for both Cr³⁺ and Cr₂O₇ ²⁻.