Abstract Microglia are innate immune cells in the central nervous system. Activation of microglia plays an important role in the processes of several neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, and HIV dementia. Activated microglia can produce various proinflammatory cytokines and nitric oxide (NO), which may exert neurotoxic effects. Inhibition of microglia activation may alleviate neurodegeneration under these conditions. To search for the novel therapeutic agents against neuroinflammatory diseases, we have screened a series of flavonoid compounds using a cell-based assay. Our studies showed that fisetin markedly suppressed the production of tumor necrosis factor (TNF)-α, NO, and prostaglandin (PG) E 2 in lipopolysaccharide (LPS)-stimulated BV-2 microglia cells or primary microglia cultures. Fisetin also inhibited the gene expression of TNF-α, interleukin (IL)-1β, cyclooxygenase (COX-2) and inducible nitric oxide synthase (iNOS) at both mRNA and protein levels. Fisetin significantly suppressed IκB degradation, nuclear translocation of NF-κB, and phosphorylation of p38 mitogen-activated protein kinase (MAPKs) in the LPS-stimulated BV-2 microglia cells. In addition, fisetin reduced cytotoxicity of LPS-stimulated microglia toward B35 neuroblastoma cells in a co-culture system. These results indicate that fisetin has a strong anti-inflammatory activity in brain microglia, and could be a potential therapeutic agent for the treatment of neuroinflammatory diseases.