A new colorimetric method based on the bleaching of the iodoplatinate ion has been developed for fast and easy determination of γ-glutamyl- S-ethenyl-cysteine (GEC) in narbon vetch ( Vicia narbonensis L.) seeds. The calibration curve showed a good correlation ( r 2 = 0.9959) between absorbance and GEC amounts from 5.5 to 33 μg (10–59.78 μmol/L). The limits of detection and quantification were 1.16 and 3.55 μmol/L, respectively, and no significant interferences from other sulfur-containing compounds were observed. The method showed excellent repeatability (relative standard deviation RSD = 0.28%), reproducibility (RSD = 4.4%), and accuracy (94%). Determination of GEC in 20 narbon vetch accessions yielded values that were in agreement with those reported previously using capillary electrophoresis and high-performance liquid chromatography methods. The method could be especially valuable for determination of GEC during the process of production of new low-GEC narbon vetch varieties.