A rapid method for determining diethylene glycol (DEG) in toothpaste based on the use of ultra-performance liquid chromatography (UPLC) coupled to time-of-flight mass spectrometry (TOF-MS) has been developed. The method has been validated in toothpaste samples spiked at different levels, 0.005, 0.1 and 5%, obtaining satisfactory recoveries (74-98%) and relative standard deviations (<4%). Quantification was carried out by using matrix-matched standards calibration. The developed method was applied to several types of toothpaste, making identification and quantification of DEG and other polyethylene glycols (PEG) feasible with very little sample manipulation, as only extraction with water is required. The excellent sensitivity of TOF-MS analysis performed in full-scan acquisition mode allowed the determination of DEG at concentration levels as low as 0.005% in samples and its reliable identification via the mass accuracy measurements provided by this instrument (<5 ppm).