Chloroform is the most important product of the chlorination of water with a concentration higher than that of other trihalomethanes (THMs) produced. The presence of THMs in water causes harm to the health of the people who use that water. Herein, a simple cotton pad-based sensing device was introduced and the conditions were optimized for the detection and determination of chloroform in different water samples. Factorial design was employed for the identification of the significant factors in chloroform assays based on the proposed method. The results showed that heating and purging did not have a significant effect on the color variations of the cotton pad sensor. The detection limit and dynamic linear range for the proposed method were 0.0083 mg L⁻¹ and 0.0087–1.52 mg L⁻¹, respectively. The sensitivity of the method was lower than that of other optical and Fujiwara-based methods used in the determination of THMs. The sensitivity of a method can be related to the simultaneous trapping and preconcentration of the analyte in the designed apparatus. The method was successfully applied to the analysis of drinking and pool water samples. The method is a simple, fast and low-cost method for checking water quality in terms of the presence of THMs.