Canonical and noncanonical nuclear factor kappaB (NF-kappaB) signaling are the two basic pathways responsible for the release of NF-kappaB dimers from their inhibitors. Enhanced NF-kappaB signaling leads to inflammatory and proliferative diseases; thus, inhibitory pathways that limit its activity are critical. Whereas multiple negative feedback mechanisms control canonical NF-kappaB signaling, none has been identified for the noncanonical pathway. Here, we describe a mechanism of negative feedback control of noncanonical NF-kappaB signaling that attenuated the stabilization of NF-kappaB-inducing kinase (NIK), the central regulatory kinase of the noncanonical pathway, induced by B cell-activating factor receptor (BAFF-R) and lymphotoxin beta receptor (LTbetaR). Inhibitor of kappaB (IkappaB) kinase alpha (IKKalpha) was previously thought to lie downstream of NIK in the noncanonical NF-kappaB pathway; we showed that phosphorylation of NIK by IKKalpha destabilized NIK. In the absence of IKKalpha-mediated negative feedback, the abundance of NIK increased after receptor ligation. A form of NIK with mutations in the IKKalpha-targeted serine residues was more stable than wild-type NIK and resulted in increased noncanonical NF-kappaB signaling. Thus, in addition to the regulation of the basal abundance of NIK in unstimulated cells by a complex containing tumor necrosis factor receptor-associated factor (TRAF) and cellular inhibitor of apoptosis (cIAP) proteins, IKKalpha-dependent destabilization of NIK prevents the uncontrolled activity of the noncanonical NF-kappaB pathway after receptor ligation.