•Indole-3-carbaldehyde (I3A) is a biomolecule inducing defense responses in plants.•The first comprehensive quantitative data collection of I3A in food was reported.•I3A reacts with the hydrazine group of carbidopa giving a yellow colored aldazine.•A simple, safe, and low-cost assay was assessed in ethanolic extract of cabbage.•LC-MS confirmed the formation of yellow aldazine in ethanolic extract of cabbage. A colorimetric assay was developed for detection of indole-3-carbaldehyde (I3A), a bioactive compound particularly abundant in vegetables of the Brassicaceae family, inducing plant defense responses and ensuring the human intestinal barrier integrity. The assay is based on the selective condensation reaction between the hydrazine group of carbidopa, used as a derivatizing agent, and the aldehyde group of I3A, generating a colored aldazine with a characteristic absorbance around 415 nm. As proof of concept, among the several natural sources here reported in a quantitative data collection, the colorimetric assay was assessed in white heart cabbage ethanolic extract. I3A content was determined by LC-MS analysis, which confirmed the colored aldazine formation in matrix. The LOD (10.4 ± 0.3 μM) obtained by the calibration curve from the visible absorbance (m = 565 ± 16 M−1, avRSD % 6.5, R2 = 0.9960) indicates that this simple, safe, and low-cost method could be beneficial also for current methodologies for identification and quantification of I3A in foods. Display omitted