Global population growth, climate change, and the environmental impact of livestock production have accelerated the search for sustainable and efficient protein sources. Fruiting bodies (mushrooms) and mycelial biomass have emerged as promising alternatives due to their high nutritional quality, low ecological footprint, and compatibility with circular bioeconomy principles. This review highlights the nutritional, biotechnological, and environmental aspects of fungal proteins obtained from both fruiting bodies and mycelial biomass of Basidiomycetes. Emphasis is placed on amino acid composition, protein digestibility, and advances in cultivation and fermentation systems for large-scale production. Submerged and solid-state fermentation processes are analyzed in terms of scalability, resource efficiency, and integration with agro-industrial residues for sustainable bioprocessing. Comparative analyses reveal that mycelial biomass production achieves high protein yields with significantly reduced land, water, and energy requirements compared to conventional protein sources. Emerging fungal species such as Schizophyllum commune and Auricularia polytricha demonstrate strong potential for producing protein-rich mycelia applicable to functional and plant-based foods. Finally, the review discusses current technological innovations, regulatory frameworks, and market perspectives that position fungal biomass as a strategic component in the ongoing global protein transition.