Nitrite ions (NO2-) are Type A inorganic contaminants, and they exert significant effects and greatly threaten human health and the environment. In addition, excessive intake of this anion might cause harmful diseases. In this study, we have successfully developed a novel hydrogel-based chemosensor for sensing nitrite ions. We first synthesized a hydrogel, 2-naphthylacetic acid-L-phenylalanine-L-phenylalanine (Nap-FF), which was based on a naphthyl acetic moiety made by a liquid-phase synthetic strategy and then investigated its characteristics and the conditions required for hydrogelation. To detect nitrite ions affecting water quality, facile synthesis of the Nap-FF hydrogel and sulfanilamide were used. Then, based on the Griess mechanism for the reaction between NO2- and NapFF-sulfanilamide, a colorimetric Griess-based reaction was performed on the hydrogel. The results confirmed visually that our chemosensor showed extremely high sensitivity to nitrite, and a significant color change from transparent to yellow/orange was observed. Correspondingly, a linear relationship was observed between the NO2- concentration and the color difference value (ΔE*), the limit of detection (LOD) was 0.72 μM and the working concentration for NO2- ranged from 8.3 μM to 16.7 mM. Furthermore, interference testing of this sensor confirmed its high selectivity toward NO2- in the presence of environmentally relevant competing ions.