Chemical Oxygen Demand (COD) is a critical parameter in wastewater quality monitoring, as it quantifies the oxygen required to chemically oxidize both organic and inorganic constituents. However, the standard closed reflux method is constrained by prolonged analysis times, the utilization of hazardous reagents, and the necessity of specialized laboratory equipment. This study details the design and performance evaluation of a portable COD analyzer utilizing UV254 absorbance to provide a rapid and simplified estimation approach. The developed system integrates a 254 nm UVC Light Emitting Diode (LED) light source, a GUVA-S12SD detector, an Analog-to-Digital Converter (ADS1115 ADC), an ESP32-C3 microcontroller, a 128×64 OLED display, a 5 mm quartz cuvette, and a lithium-ion battery power supply. Experimental validation was conducted using domestic wastewater, alongside synthetic soy sauce and syrup wastewater, prepared across a concentration gradient of 20%, 40%, 60%, 80%, and 100%. The absorbance values obtained from the portable device were benchmarked against a Brolight BIM-6002 ultraviolet-visible (UV-Vis) spectrophotometer and validated against reference laboratory COD values determined via the APHA 5220C standard method. The results demonstrate that the portable device accurately tracks UV254 absorbance variations, exhibiting a strong correlation with the benchmark spectrophotometer (R2 = 0.7976). A second-order polynomial regression model correlating the portable absorbance with the reference COD yielded the empirical equation COD = 18037A2 + 2882.3A - 204.98 (R2 = 0.8144), while the spectrophotometer-based model achieved an R2 of 0.8241. Performance analysis revealed that while the universal model exhibited a Mean Absolute Error (MAE) of 291.27 mg/L and a Root Mean Square Error (RMSE) of 347.07 mg/L, matrix-specific models substantially enhanced accuracy, reducing the MAE and RMSE to 27.73 mg/L and 48.12 mg/L, respectively. These findings underscore the potential of UV254 absorbance as a robust surrogate parameter for rapid COD screening, though matrix-specific calibration remains imperative to ensure high-precision quantification.