The transcription factor NF-κB has an essential role in inflammation in endothelial cells. Endothelial nitric oxide synthase (eNOS)-derived nitric oxide (NO) prevents vascular inflammation. However, the molecular mechanism underlying NF-κB-mediated regulation of eNOS expression has not been clearly elucidated. We here found that NF-κB-activating stimuli, such as lipopolysaccharide, tumor necrosis factor-α (TNF-α), and interleukin-1β, suppressed eNOS mRNA and protein levels by decreasing mRNA stability, without affecting promoter activity. TNF-α-mediated suppression of eNOS expression, mRNA stability, and 3′-untranslated region (3′UTR) activity were inhibited by NF-κB inhibitors and Dicer knockdown, but not by p38 MAPK and MEK inhibitors, suggesting the involvement of NF-κB-responsive miRNAs in eNOS expression. Moreover, TNF-α increased MIR155HG expression and promoter activity as well as miR-155 biogenesis, and these increases were blocked by NF-κB inhibitors. Transfection with antagomiR-155 blocked TNF-α-mediated suppression of eNOS 3′UTR activity, eNOS mRNA and protein levels, and NO and cGMP production. These data provide evidence that NF-κB is a negative regulator of eNOS expression via upregulation of miR-155 under inflammatory conditions. These results suggest that NF-κB is a potential therapeutic target for preventing vascular inflammation and endothelial dysfunction induced by suppression of miR-155-mediated eNOS expression.