Despite the use of toxic and not environmentally-friendly chemicals, some proteins derived from animal or microbial sources have been investigated as novel green flame retardants for cotton fabrics. In particular, phosphorus- and sulphur-rich proteins (i.e. caseins and hydrophobins) have been homogeneously deposited on cotton fabrics starting from protein aqueous suspensions/solutions. These surface treatments, based on the use of species able to favour the dehydration of cellulose instead of its depolymerization, have strongly enhanced the production of a thermally stable carbonaceous structure (char), hence significantly enhancing the flame retardancy of the fabrics, as assessed by thermogravimetry and flammability tests.