<p>Aquaculture is the fastest-growing food production sector, and for sustainable development, fish feed alternatives to traditional ingredients such as fishmeal and soybean meal are essential. Given their nutrient content and economic and ecological advantages, agricultural byproducts are excellent alternatives. Nevertheless, many of these byproducts are limited in their use in aquafeeds due to anti-nutritional factors, variability in nutrient composition, and digestibility problems. The bioaugmentation of agricultural byproducts through fermentation has been highlighted as a dynamic biotechnological strategy to improve their nutritional value by increasing protein bioaccessibility, minimizing the anti-nutritional factors, and benefiting the gut health of fish. Various fermentation techniques, including solid-state fermentation and submerged fermentation, have been employed to optimize the functional properties of feed ingredients, leading to improved feed conversion efficiency and fish growth performance. This review highlights the potential of fermentation to valorize agricultural byproducts into high-quality aquafeeds, covering different fermentation methods, microbial strains used, and nutritional and immunological benefits for fish. More importantly, this review highlights the economic feasibility, challenges, and potential large-scale applications of fermentation technology in aquaculture. Fermentation-based feed strategies can facilitate the valorization of agricultural residues, reduce dependency on finite resources, and support a circular bioeconomy in aquaculture. Further studies are needed to optimize fermentation protocols, choose effective microbial consortia, and explore the long-term advantages of fermented feed ingredients in different fish species.</p>