We present a colorimetric method based on gold nanoparticle aggregation to detect and quantify poly-diallyldimethylammonium chloride (poly-DADMAC), a common water treatment polyelectrolyte, in river water. The protocol developed has an excellent linear range between 10 and 100 μg L⁻¹ (R = 0.99), with a lower limit of detection of 0.54 μg L⁻¹ and a lower limit of quantification of 1.5 μg L⁻¹. The method has excellent intermediate precision (0.1–0.7%), relatively quick analysis times, requires no extraction or derivatization methods, and is robust and rugged. Results of spiked river water samples collected from the Umgeni River located in the province of KwaZulu-Natal, South Africa show that the method can detect low levels of poly-DADMAC in environmental matrices.