We report a selective, sensitive and fast liquid chromatography/tandem mass spectrometry (LC/MS/MS) method for the determination of diallyldimethylammonium chloride (DADMAC) in water. Hydrophilic interaction liquid chromatography (HILIC) was used to avoid ion‐pairing reagents, which are generally employed to retain cationic compounds. The complementary information obtained in a triple quadrupole mass spectrometer and in an ion trap Orbitrap has been used to study the fragmentation of the DADMAC cation [M]+ and for the correct assignment of the products ions. The HILIC/MS/MS method developed, using electrospray ionization in positive ion mode and selected reaction monitoring (SRM) acquisition mode, led to a reliable determination and confirmation of the DADMAC cation in water samples down to 50 ng L−1. The low detection limit achieved, in combination with the absence of matrix effects, allowed the direct analysis of samples without any pretreatment, preconcentration or clean‐up step. DADMAC was determined in samples collected in a drinking water treatment plant (DWTP) in Barcelona (Spain) and it was found in the influent at the µg L−1 level. Copyright © 2010 John Wiley & Sons, Ltd.