Osteoarthritis (OA) is one of the major joint diseases, and the synovial inflammation is involved in the pathogenesis and progression of OA. Glucosamine (GlcN) is widely used as a dietary supplement for OA, and is expected to exert the antiinflammatory action in OA. However, the detailed mechanism for the antiinflammatory action of GlcN remains poorly understood. In this study, to elucidate the molecular mechanism involved in the GlcN-medicated regulation of synovial cell activation, we comprehensively analyzed the effect of GlcN on the gene expression using a human synovial cell line MH7A by DNA microarray. The results indicated that GlcN significantly downregulates the expression of 187 genes (≤1/1.5-fold) and upregulates the expression of 194 genes (≥1.5-fold) in IL-1β-stimulated MH7A cells. Interestingly, pathway analysis indicated that among the 10 pathways into which the GlcN-regulated genes are categorized, the 4 pathways are immune-related. Furthermore, GlcN suppressed the expression of proinflammatory cytokine genes (such as IL-6, IL-8, IL-24 and TNF-α genes). In addition, GlcN-mediated O-GlcNAc modification was involved in the downregulation of TNF-α and IL-8 genes but not IL-6 and IL-24 genes, based on the effects of alloxan, an O-GlcNAc transferase inhibitor. Thus, GlcN likely exerts an antiinflammatroy action in OA by suppressing the expression of proinflammatory cytokine genes in synovial MH7A cells by O-GlcNAc modification-dependent and -independent mechanisms.