Laetiporus sulphureus (Bull.: Fr.) Murr. was studied to determine the nutritional value, bioactive compounds, in vitro antioxidants, and antimicrobial and antitumor activities. The studied mushroom is a rich source of carbohydrates and proteins. Mannitol and trehalose were the main free sugars. In addition, the polyunsaturated fatty acids α-, γ- and δ-tocopherols were found. Oxalic and citric acids were the most abundant organic acids; cinnamic and p-hydroxybenzoic acids were quantified in the methanolic extract and could be related to the antioxidant properties. It was the polysaccharidic extract that exhibited higher antioxidant and antimicrobial activities, indicating that the compounds present in this extract possess stronger bioactivity. Only the polysaccharidic extract revealed antiproliferative activity in human tumor cell lines. In addition, a suitable model system with chicken pâté was developed to test the antimicrobial preserving properties of L. sulphureus. The methanolic extract was used to examine in situ preserving properties against Aspergillus flavus and demonstrated excellent preserving potential.