Here, N-acetyl-d-glucosamine (GlcNAc), the monomer composing the second most abundant biopolymer, chitin, was efficiently converted into 5-hydroxymethylfurfural (5-HMF) using ionic liquid (IL) catalysts in a water/dimethyl sulfoxide (DMSO) mixture solvent. Various reaction parameters, including reaction temperature and time, DMSO/water mass ratios and catalyst dosage were optimized. A series of ILs with different structures were analyzed to explore their impact on GlcNAc conversion. The substrate scope was expanded from GlcNAc to d-glucosamine, chitin, chitosan and monosaccharides, although 5-HMF yields obtained from polymers and other monosaccharides were generally lower than those from GlcNAc. Moreover, the IL N-methylimidazolium hydrogen sulfate ([Hmim][HSO]) exhibited the best catalyst performance (64.6% yield) when GlcNAc was dehydrated in a DMSO/water mixture at 180 °C for 6 h without the addition of extra catalysts. To summarize, these results could provide knowledge essential to the production of valuable chemicals that are derived from renewable marine resources and benefit biofuel-related applications.