Plastic pollution and food waste are two pressing global challenges that require immediate attention and innovative solutions. In this study, we address these challenges by upcycling brewers’ spent grain (BSG) into biodegradable composite films. BSG, a by-product of the beer brewing process, is commonly discarded in landfills or used as animal feed. By utilizing BSG as a raw material for biodegradable films, we simultaneously reduce waste and decrease plastic pollution. To create the composite films, we employed poly(vinyl alcohol) (PVA) and glycerol as binder materials, along with hexamethoxymethylmelamine (HMMM) as a water-repelling agent. By varying the ratios of these components, we investigated the effects on film properties. Our characterization included assessing moisture uptake and tensile properties. The results revealed that the practical BSG content in the films was 20−60 wt%. Films with this composition exhibited a balance between moisture absorption and mechanical strength. The addition of glycerol improved the flexibility and toughness of the films, while HMMM reduced moisture absorption, enhancing their water resistance. This study contributes to the development of sustainable materials by showcasing the potential of upcycling BSG into valuable biodegradable films. By transforming food waste into useful applications, we reduce environmental burdens and promote a circular economy. Further research is warranted to explore the potential applications and optimize the properties of BSG-based composites.