Considering the significance of its demand around the world, the accurate determination of fish freshness with a simple and rapid procedure has become an interesting issue for the fishing industry. Hence, we aimed to fabricate a new optical pH sensor based on a polyelectrolyte (PEC) membrane of pectin–chitosan and the active material chromoionophore ETH 5294. A trial-and-error investigation of the polymer compositions revealed that the optimum ratio of pectin to chitosan was 3:7. With an optimum wavelength region (λ) at 610 nm, the constructed sensor was capable of stable responses after 5 min exposure to phosphate-buffered solution. Furthermore, the obtained sensor achieved optimum sensitivity when the PBS concentration was 0.1 M, while the relative standard deviation values ranged from 2.07 to 2.34%, suggesting good reproducibility. Further investigation revealed that the sensor experienced decreased absorbance of 16.67–18.68% after 25 days of storage. Employing the optimum conditions stated previously, the sensor was tested to monitor fish freshness in samples that were stored at 4 °C and ambient temperature. The results suggested that the newly fabricated optical sensor could measure pH changes on fish skin after 25 h storage at room temperature (pH 6.37, 8.91 and 11.02, respectively) and 4 °C (pH 6.8, 7.31 and 7.92, respectively).