This comprehensive review explores how filamentous fungi—like Aspergillus, Pleurotus, and Flammulina—can sustainably replace soy and fishmeal in broiler chicken diets. Mushroom fruiting bodies, mycelia, and even spent substrate (a byproduct of mushroom farming) were tested. Results from multiple feeding trials showed either neutral or positive effects on broiler growth, immune response, gut health, and meat quality. Bioactive compounds like β-glucans, chitin, and phenolics played key roles by supporting beneficial gut microbes and enhancing nutrient uptake. Though not all fungi had the same impact, using mushroom waste as feed could reduce environmental costs and support circular agriculture.