A high-performance chromatographic method was developed to determine the fungicide kresoxim-methyl. Off-line photochemical derivatization was used to induce the formation of a stable and fluorescent product since the fungicide does not present natural fluorescence. Intense fluorescence at 370/430 nm was achieved by treating the analyte in solution at pH 6 to ultraviolet light for 45 s. The chromatographic conditions included isocratic elution with 50/50% (v/v) acetonitrile/water and the photochemical product appeared at a retention time of 7.2 min. The short and long term stabilities of the photoproduct were evaluated and variation of less than 5% was achieved. The limits of detection in water samples and in grapes samples were 0.019 mg kg−1 and 0.065 mg kg−1 of kresoxim- methyl residue, respectively. The linear response covered three orders of magnitude up to 10.6 mg kg−1 of kresoxim-methyl. The robustness was evaluated through a Box–Behnken experimental design showing the insignificance of all factors and their interactions. The potential interference of tebuconazole for the determination of kresoxim-methyl was studied. The use of the dispersive liquid-liquid microextraction (DLLME) allowed recoveries between 80% and 101% depending on concentration with the minimum generation of waste products.