B cell activation factor of the TNF family (BAFF) activates noncanonical nuclear factor κB (NF-κB) heterodimers that promote B cell survival. We show that although MALT1 is largely dispensable for canonical NF-κB signaling downstream of the B cell receptor, the absence of MALT1 results in impaired BAFF-induced phosphorylation of NF-κB2 (p100), p100 degradation, and RelB nuclear translocation in B220⁺ B cells. This corresponds with impaired survival of MALT1^−/− marginal zone (MZ) but not follicular B cells in response to BAFF stimulation in vitro. MALT1^−/− MZ B cells also express higher amounts of TRAF3, a known negative regulator of BAFF receptor–mediated signaling, and TRAF3 was found to interact with MALT1. Furthermore, phenotypes associated with overexpression of BAFF, including increased MZ B cell numbers, elevated serum immunoglobulin titers, and spontaneous germinal center formation, were found to be dependent on B cell–intrinsic MALT1 expression. Our results demonstrate a novel role for MALT1 in biological outcomes induced by BAFF-mediated signal transduction.