Loss of Bruton’s tyrosine kinase (Btk) function results in mouse Xid disease characterized by a reduction in mature B cells and impaired humoral immune responses. These defects have been mainly attributed to impaired BCR signaling including reduced activation of the classical NF-κB pathway. In this study we show that Btk also couples the receptor for B cell-activating factor (BAFF) of the TNF family (BAFF-R) to the NF-κB pathway. Loss of Btk results in defective BAFF-mediated activation of both classical and alternative NF-κB pathways. Btk appears to regulate directly the classical pathway in response to BAFF such that Btk-deficient B cells exhibit reduced kinase activity of IκB kinase γ-containing complexes and defective IκBα degradation. In addition, Btk-deficient B cells produce reduced levels of NF-κB2 (p100) basally and in response to stimulation via the BCR or BAFF-R, resulting in impaired activation of the alternative NF-κB pathway by BAFF. These results suggest that Btk regulates B cell survival by directly regulating the classical NF-κB pathway under both BCR and BAFF-R, as well as by inducing the expression of the components of alternative pathway for sustained NF-κB activation in response BAFF. Thus, impaired BCR-and BAFF-induced signaling to NF-κB may contribute to the observed defects in B cell survival and humoral immune responses in Btk-deficient mice.