This paper presents the design and fabrication of an automatic colorimetric sensor for on-site determination of nitrite in aquaculture. The colorimetric analysis of nitrite is based on the API nitrite test kit reagents, and the color intensity of nitrite solution is detected using a low-cost color sensor. The fabricated sensing system uses off-the-shelf components and has a compact housing that makes it low-cost and highly portable. This sensor is capable of detecting nitrite in water up to 5 mg/L. Experimental results showed that the nitrite concentrations estimated by the designed sensor were almost comparable to those obtained with the commercial spectrophotometer. With integrated wireless communications capabilities, it helps farmers to respond promptly to adverse changes in pond water parameters and to minimize losses. The colorimetric sensing platform designed in this paper can be applicable to more chemical analysis by establishing the calculation model according to the absorption spectra of the new reagent solution.