Background: The rainbow trout fish is susceptible to spoilage due to its high content of unsaturated fatty acids. It should be kept at low temperature to reduce microbial, enzymatic, and oxidation reactions. The purpose of this study was to design a packaging that contains a pH indicator for monitoring freshness of the rainbow trout fish during storage at refrigerator. Methods: The indicator contained agarose as the carrier, bromocresol green as pH indicator, and silica as surface provider. It was covered by polypropylene film and attached inside the package. Freshness of the trout stored in the refrigerator was assessed by chemical (total volatile basic nitrogen and pH) and microbiological (total viable count) methods. Results: The pH of fish gradually decreased after the third day since color of the indicator changed from yellow to green on day 3 and then to blue on day 6. The indicator's response was correlated with changes in the microbial population and also with levels of total volatile basic nitrogen and pH. The results showed that the designed indicator was sensitive to different pH levels and could be applied as part of the intelligent packaging system. Conclusion: The freshness indicator worked well before the expiry date of fish, which makes it suitable for food quality assessment. So, this indicator can be used for real-time monitoring of packaged fish freshness