The current study aimed to develop and evaluate a biodegradable edible coating derived from Pleurotus ostreatus (oyster mushroom) polysaccharides, incorporating green-synthesized silver nanoparticles (AgNPs), for the postharvest preservation of fruits. Glycerol was used as a plasticizer in the film formulation, and the composite films were characterized using UV–Vis spectroscopy (peak at ~420 nm), X-ray diffraction (XRD), and scanning electron microscopy (SEM) analysis. The coating was applied to tomatoes and guavas and evaluated over 14 days at 25 °C for its impact on weight loss, firmness, decay incidence, microbial load, sensory attributes, vitamin C retention, and biodegradability. The AgNP-enhanced films exhibited improved physicochemical properties, including a reduction in water vapor permeability (from 3.75 to 2.89 × 10⁻¹¹ g/m·s·Pa) and enhanced antimicrobial activity, with inhibition zones of 16.5 mm for E. coli and 19.5 mm for Staphylococcus aureus. After 14 days, the coated fruits showed significantly lower weight loss (5.1 ± 0.5%), decay incidence (7%), and microbial load (3.5 CFU/g) compared to both uncoated and polyethylene-wrapped samples. Sensory evaluations showed high acceptability (scores >8.0), and nutritional quality was maintained, with vitamin C retention at 80%. The coating demonstrated approximately 90% biodegradability within 30 days under soil burial conditions and met safety standards regarding nanoparticle migration. These findings highlight the potential of mushroom-based AgNP films as a sustainable, safe, and effective alternative to conventional plastic packaging for extending the shelf life and preserving the quality of fresh produce.