Disinfection plays a crucial role in the production of potable water, the maintenance of swimming pools, the sanitation of food industry facilities, and medical and hospital environments. Among disinfection methods, chlorination involves the use of gaseous chlorine, chlorine dioxide, calcium hypochlorite, or, most commonly, sodium hypochlorite. It is important to monitor the concentration of free residual chlorine, i.e., the amount of disinfectant remaining in water after treatment. Residual free chlorine can react with natural organic matter to form disinfection by-products classified as carcinogens. The ISO 7393-2 standard detects free residual chlorine by its reaction with N,N-diethyl-p-phenylenediamine (DPD), producing a pink complex with intensity proportional to the residual chlorine concentration. The method requires benchtop instrumentation and trained personnel, making routine monitoring expensive and time-consuming. A paper-based colorimetric sensor was developed as a low-cost, rapid, portable, and user-friendly alternative. The sensor leverages color change resulting from the DPD reaction and allows naked-eye detection. Quantitative analysis can be performed in only 3 min by capturing images of the sensor using a smartphone, allowing for precise colorimetric evaluation. This approach achieved a detection limit of 12 μM for sodium hypochlorite, making it a promising tool for routine water quality monitoring applications.