Abstract NF-κB is a family of important transcription factors that modulate immunity, development, inflammation, and cancer. The biological activity of NF-κB is subjected to various spatial and temporal regulations. Bioinformatics analysis predicts that CITED2 is topologically close to NF-κB in the protein interaction networks. In this study, we show that ectopic expression or knockdown of CITED2 attenuates or potentiates, respectively, the expression of NF-κB–responsive genes. Mechanistically, CITED2 constitutively localizes inside the nucleus and interacts specifically with the coactivator p300. This prevents p65 from binding to p300, impairs p65 acetylation, and attenuates p65 binding to its cognate promoters. Furthermore, LPS induces CITED2 expression via NF-κB in macrophages. CITED2 sensitizes cells to TNF-α–induced apoptosis. Collectively, this study identifies CITED2 as a novel regulator of NF-κB in the nucleus, which reveals a negative feedback mechanism for NF-κB signaling.