Forkhead transcription factor3 (Foxp3) is critical for generating CD4⁺CD25⁺ regulatory T cells. However, its role in microglia has not been identified. Here, we show that Foxp3 is expressed in microglia and is upregulated upon activation. In Foxp3 mutant mice (Foxp3^sf), microglia release higher levels of inflammatory cytokines and mediators such as NO, MCP-1, CXCL10, and ROS upon liposaccharide treatment than the wild type, while TNF-α and IL-1β were not significantly different between wild and mutant microglial cells. In addition, Foxp3 silencing enhances inflammatory responses, suggesting that the major role of Foxp3 in microglia is that of a repressor of activation. Similarly, Foxp3 overexpression reduces inflammatory responses in microglia. We also demonstrate that Foxp3 interacts directly with NF-κB and modulates its transcriptional activities. These findings point to the importance of Foxp3 in NF-κB mediated inflammatory responses in microglia. © 2010 Wiley-Liss, Inc.