Regulatory B cells (Bregs) potently suppress immune disorders, including allergic contact hypersensitivity (CHS). IKKβ overactivation is prominent in various inflammatory diseases. However, its effect on Bregs has not been defined. This study is to investigate the new regulator and inhibitory mechanism of Bregs. Ikkβ transgenic mice with a Cys46 mutation, resulting in increased IKKβ activation, were employed for analysis. IL-10-competent CD9 Bregs were expanded in Ikkβ mice and B cell specific-Ikkβ mutation mice. Ikkβ mutant CD9 Bregs had stronger suppressive effects on CD4 and CD8 T cells in vitro and CHS responses in vivo. The inhibitory CD9 Bregs from Ikkβ mice were characterized by upregulated Neuropilin 2 (Nrp2) and IL-10 in comparison with that of Ikkβ mice. Interestingly, increased expression of Nrp2 was observed in CD9 Bregs compared with that of CD9 B cells in wild-type mice. The suppressive activity of wild-type CD9 Bregs in vitro was attenuated by inhibition of Nrp2 on Bregs or silencing its ligand Sema3f on CD4 T cells. Our findings delineate a distinct role of IKKβ activation in enhancing Bregs to disturb the immune balance. It identifies Nrp2 as a novel regulatory molecule of Bregs that partly contributes to B cell-mediated immune tolerance.