A simple and sensitive colorimetric detection method for hexavalent chromium (Cr(VI)) was described based on ascorbic acid (AA)-capped silver nanoparticles (AgNPs). The cross-linking between AA and Cr(III), the reductive product of Cr(VI), resulted in the aggregation of AA-capped AgNPs, and the color change from yellow to red. Control experiments with the addition of over 15 other metal ions (Cr³⁺, Pb²⁺, Zn²⁺, Hg²⁺, Cd²⁺, Co²⁺, Cu²⁺, Al³⁺, Ni²⁺, Ag⁺, Mn²⁺, Ba²⁺, Fe³⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺) did not result in a distinct change in the color or in the spectrum of the suspension, indicating that these ions do not interfere with the colorimetric determination of Cr(VI). The detection concentration of Cr(VI) ranged from 7.0 × 10⁻⁸ to 1.84 × 10⁻⁶ M, and the detection limit was 5 × 10⁻⁸ M (3δ). The performance of the sensor was successfully evaluated with water samples spiked with Cr(VI), indicating that this convenient and sensitive technique offers great promise for onsite environmental monitoring of Cr(VI).