Articles in recent years have described two separate and distinct NF-κB activation pathways that result in the differential activation of p50- or p52-containing NF-κB complexes. Studies examining tumor-necrosis factor receptor-associated factors (TRAFs) have identified positive roles for TRAF2, TRAF5, and TRAF6, but not TRAF3, in canonical (p50-dependent) NF-κB activation. Conversely, it recently was reported that TRAF3 functions as an essential negative regulator of the noncanonical (p52-dependent) NF-κB pathway. In this article, we provide evidence that TRAF3 potently suppresses canonical NF-κB activation and gene expression in vitro and in vivo. We also demonstrate that deregulation of the canonical NF-κB pathway in TRAF3-deficient cells results from accumulation of NF-κB-inducing kinase (NIK), the essential kinase mediating noncanonical NF-κB activation. Thus, our data demonstrate that inhibition of TRAF3 results in coordinated activation of both NF-κB activation pathways.