The synthetic chemical bisphenol A (BPA) is widely used in food containers, water bottles, and other items. However, problems occur when it leaches into the food and water. Being a strong endocrine disruptor, it may change a number of bodily functions or trigger estrogen-like activities that have a variety of detrimental consequences on health and can potentially cause carcinogenesis and mutagenesis. Due to this, it is important to determine the presence of BPA in water samples. Traditional methods for detecting BPA use highly sensitive tools like inductively coupled plasma-mass spectrometry, mass spectrometry, or other alternative spectrophotometry analysis. These tools can reliably examine the contaminants with high sensitivity. However, these instruments are not ideal for quick evaluation due to their high cost and time consumption, which is uneconomical for on-site and real-time monitoring. Hence, this research investigated an available quantitative technique using the paper-based analytical colorimetric method for BPA detection using functionalized chitosan. The reaction was investigated at different BPA concentrations and modified fabrication methods of chitosan-paper-based analytical devices. The research also determined the optimal parameters for BPA detection under selective conditions and its performance at higher BPA concentrations, i.e., the concentration of sulfamethoxazole, the concentration of sodium nitrite, and the effect of pH. Quantitative detection of BPA was accomplished using a smartphone and ImageJ software. Based on the outcome, the percentage of similarity between the modified Chitosan paper-based analytical device fabrication method and the Karrat’s reported method is about 35% after considering the key parameters of fabrication and performance metrics.