Smart packaging containing red poppy anthocyanins was developed for monitoring fish freshness. Whey protein isolate (WPI) and chitin nanofiber (CNF) were used as the matrix materials, with red poppy anthocyanins (RPAs) acting as color-sensing agents. The RPAs were uniformly dispersed in the biopolymer matrix, resulting in a smooth surface. The incorporation of RPAs increased the flexibility and strength of the indicator, while decreasing water vapor permeability, water solubility, and swelling index. The WPI/CNF/RPAs indicator showed high sensitivity to acid, base, and ammonia, and exhibited noticeable color changes from brownish-yellow to gray at 4°C, indicating fish spoilage. Simulated tests demonstrated that pH-sensitive indicators can visually determine fish freshness before consumption. Smart packaging films, made from biodegradable materials, offer a way to monitor food spoilage without opening the package, reducing waste and improving food quality. The main components of degradable packaging films include polysaccharides, proteins, and lipids. WPI, a byproduct of cheese production, is used in food packaging films due to its excellent film-forming ability, biodegradability, and high nutritional value. However, its hydrophilic nature and low tensile strength limit its use. To overcome these limitations, biopolymer compounds or nanostructures such as chitin nanofibers (CNF) can be incorporated into the film structure. CNF, a crystalline form of chitin, is a cost-effective and easily produced nanofiller with high mechanical strength and antimicrobial properties. Natural pigments, such as anthocyanins, are ideal indicators for smart packaging due to their non-toxic, renewable, and easily decomposable nature. Anthocyanins from red poppy (RP) have high antioxidant and antimicrobial properties. This study tested the concept of using WPI/CNF-based films containing RP for both protecting and monitoring food spoilage during storage. The films were evaluated for their barrier, protective, physical-mechanical, and visual properties. The results showed that the WPI/CNF/RPAs indicator could be used as an intelligent biodegradable film for monitoring fish freshness.Smart packaging containing red poppy anthocyanins was developed for monitoring fish freshness. Whey protein isolate (WPI) and chitin nanofiber (CNF) were used as the matrix materials, with red poppy anthocyanins (RPAs) acting as color-sensing agents. The RPAs were uniformly dispersed in the biopolymer matrix, resulting in a smooth surface. The incorporation of RPAs increased the flexibility and strength of the indicator, while decreasing water vapor permeability, water solubility, and swelling index. The WPI/CNF/RPAs indicator showed high sensitivity to acid, base, and ammonia, and exhibited noticeable color changes from brownish-yellow to gray at 4°C, indicating fish spoilage. Simulated tests demonstrated that pH-sensitive indicators can visually determine fish freshness before consumption. Smart packaging films, made from biodegradable materials, offer a way to monitor food spoilage without opening the package, reducing waste and improving food quality. The main components of degradable packaging films include polysaccharides, proteins, and lipids. WPI, a byproduct of cheese production, is used in food packaging films due to its excellent film-forming ability, biodegradability, and high nutritional value. However, its hydrophilic nature and low tensile strength limit its use. To overcome these limitations, biopolymer compounds or nanostructures such as chitin nanofibers (CNF) can be incorporated into the film structure. CNF, a crystalline form of chitin, is a cost-effective and easily produced nanofiller with high mechanical strength and antimicrobial properties. Natural pigments, such as anthocyanins, are ideal indicators for smart packaging due to their non-toxic, renewable, and easily decomposable nature. Anthocyanins from red poppy (RP) have high antioxidant and antimicrobial properties. This study tested the concept of using WPI/CNF-based films containing RP for both protecting and monitoring food spoilage during storage. The films were evaluated for their barrier, protective, physical-mechanical, and visual properties. The results showed that the WPI/CNF/RPAs indicator could be used as an intelligent biodegradable film for monitoring fish freshness.