A hydrophilic liquid chromatography-tandem mass spectrometry method was developed for determination of cyanuric acid in whey powder samples in the mass fraction range from 1.00 to 100.0 mg kg⁻¹. Cyanuric acid was extracted from whey powder with an acetonitrile–water (60 : 40, v/v) extraction solution. Separation was performed on a 3.5 μm XBridge Amide column using acetonitrile–water (90 : 10, v/v) as the mobile phase at a flow rate of 0.2 mL min⁻¹. Detection was achieved by triple quadrupole mass spectrometry using a heated electrospray ionization interface. The analysis was performed in the negative ionization mode using the SRM transition combinations of m/z 128 → 42 and m/z 128 → 85 for quantitative and qualitative detection of cyanuric acid, respectively, and m/z 134 → 44 and m/z 134 → 89 for isotope-labelled cyanuric acid (¹³C₃, ¹⁵N₃-CYA) detection, used to correct for matrix effects. The developed and optimized method was validated by determining the following validation parameters: selectivity, linearity, precision, accuracy, recovery, limits of detection and quantification and stability of cyanuric acid in whey powder extracts. The recoveries of cyanuric acid from whey powder spiked at 1.00, 2.50, 7.00, 50.0 and 100.0 mg kg⁻¹ were within the range of 96.7–107.2%, intra-day precision was ≤6.45%, inter-day precision was ≤9.13% and accuracy was below 10%. Matrix effects investigated with three different matrices were less than 15%. The limits of detection and quantification were 0.30 mg kg⁻¹ and 1.00 mg kg⁻¹, respectively. The suitability of the developed method for use in routine determination was confirmed through analysis of whey powder samples from dairy industry and supermarkets.