A simple and rapid gas chromatography (GC) method with mass spectrometry (MS) detection has been developed for the determination of methyldibromo glutaronitrile (MDBGN) in cosmetic products. The presence of this preservative in commercial cosmetic samples is prohibited since 2007 because of its allergenic properties. The analyzed products were opportunely diluted in methanol and MDBGN was separated by fused silica capillary column and detected by electron impact (EI)-MS in positive ionization mode with a total run time of 7 min. The assay was validated in the range 0.005–0.100 mg MDBGN per g of examined product with good determination coefficients (r2 ≥ 0.99) for the calibration curves. At three concentrations spanning the linear dynamic range of the calibration curves, mean recoveries were always higher than 95% for MDBGN in the tested cosmetics. This method was successfully applied to the analysis of cleansing gels, shampoo and solar waters to disclose the eventual presence of MDBGN illegally added in cosmetic products.