Background: Water quality is critically important for the global environment and public health. Polydiallyldimethylammonium chloride is cationic polyelectrolyte which acts as a flocculant and coagulant, helping to purify water from suspended insoluble particles. Despite its valuable properties, excessive levels of polydiallyldimethylammonium chloride in treated water can lead to the formation of carcinogenic nitrosamines. Existing procedures for the determination of polydiallyldimethylammonium chloride are characterized either by limits of detection higher than the maximum residue level or by the use of expensive equipment. Results: A new hydrophobic deep eutectic solvent based on a primary straight-chain alkyl amine and a natural terpenoid has been proposed for liquid-liquid microextraction procedure to determine polydiallyldimethylammonium chloride in water using smartphone-based colorimetry. The technique developed involves sequentially passing the sample through cartridges containing ion exchange resin and cotton wool to remove interferences, with the analyte retained on the cotton wool. An aqueous pyrogallol red solution is then passed through the cotton wool cartridge, resulting in the formation of an ion-pair associate with the analyte. The deep eutectic solvent is subsequently used to extract the excess pyrogallol red. The amine component promotes an alkaline medium, enhancing dye ionization and enabling the mass transfer of its anionic form into the extract phase. Under optimized conditions, the limit of detection is 10 μg L−1, calculated based on a blank test using 3σ, and the linear detection range is 30–120 μg L−1. Significance and novelty: The procedure developed is the first smartphone-based technique for the determination of PolyDADMAC that provides low limit of detection for the analyte, does not require expensive analytical equipment, materials and reagents, and can be applied for practical environmental water monitoring.