N-Acetyl glucosamine (GlcNAc) is one of the most abundant biomolecules on Earth and is cheaply available from chitin, a major component of crustaceans. The key step in the conversion of GlcNAc to high value products is the de-N-acetylation to glucosamine, in itself a valuable dietary supplement that is produced at over 29,000 tons scale per annum by chemical hydrolysis, a process that requires harsh reaction conditions and leads to side products requiring separation. Here, we report for the first time the isolation and characterisation of an enzyme, deacetylase from Cyclobacterium marinum that is able to catalyse the highly selective quantitative hydrolysis of GlcNAc to glucosamine under mild reaction conditions. The enzyme is small (38 kDa), is easily obtainable by heterologous expression in E.coli, has high turnover rates (Kcat 61 s-1), tolerates high substrate concentrations (over 100 g/L) and can be repeatedly re-used as an immobilised catalyst. When coupled with chitinase, the high selectivity of the enzyme for GlcNAc over other biomolecules allowed one-pot extraction of glucosamine from a crude solids mushroom fractions containing chitin, thus allowing for alternative production of glucosamine from non-animal sources, of benefit to consumers with crustacean allergies and vegan diets. We suggest that the deacetylase fills an important gap in the sustainable exploitation of GlcNAc and chitin.