Fast demographic growth has led to increasing interest in low-cost alternative protein sources to meet population needs. Consequently, scientific researchers have been focused on finding under-exploited sources of protein, alternative to those of animal origin. Usually plant proteins have been used for this purpose; however, most of them are not considered to be high quality due to their lack of some essential amino acids. Mushroom proteins usually have a complete essential amino acid profile, which may cover the dietetic requirements, as well as may have certain economic advantages as compared to animal and plant sources. Many mushrooms also have the ability to grow in agro-industrial waste, as submerged cultures, reaching high yields in a short period of time. Edible mushrooms can be processed to obtain a wide variety of food products enriched with high quality protein, which may have as well improved functional properties, giving them an added value. This review discusses the use of mushrooms as sustainable functional food.