SignificanceNF-κB activation pathways program cells to induce various activities including survival and inflammation. NF-κB activation requires inhibitor IκB degradation. Two of the IκB proteins, p100 and p105, are also the precursors of two NF-κB proteins, p52 and p50. The mechanism by which the p100 inhibits NF-κB remains elusive. The X-ray structure of the C-terminal domain of p100 reveals tetrameric organization and an essential function of tetramerization in sequestering four NF-κB molecules in a subunit-independent manner. Such broader nonspecific NF-κB inhibition by p100 contrasts with the functions of other IκB proteins. A host of genetic mutations linked to excessive p100 processing derail homeostatic NF-κB activity. Our work provides a molecular framework to understand homeostatic regulation of p100 function.