Loss of the regulatory mechanisms that avoid excessive or constitutive activation of NF-κB may be associated with chronic inflammatory disorders, including rheumatoid arthritis (RA). After massive sequencing of 158 regulators of the NF-κB pathway in RA patients, we focused on a scarcely known gene, ASCC1, and showed that it potently inhibits the expression of NF-κB target genes (TRAIL, TNF-α, cIAP-1, IL8) and blocks activation of a NF-κB–luciferase reporter construct in five different human cell lines. Therefore, ASCC1 may contribute to avoiding a pathologic activation of this transcription factor. A truncated variant of ASCC1 (p. S78*) was found in RA patients and control individuals. Functional in vitro studies revealed that truncation abrogated the NF-κB inhibition capacity of ASCC1. In contrast with full-length protein, truncated ASCC1 did not reduce the transcriptional activation of NF-κB and the secretion of TNF-α in response to inflammatory stimuli. We analyzed the clinical impact of p. S78* variant in 433 patients with RA and found that heterozygous carriers of this variant needed more disease-modifying antirheumatic drugs, and more patients with this genotype needed treatment with corticoids and biologic agents. Moreover, the truncated allele-carrier group had lower rates of remission compared with the full-length variant carriers. Overall, our findings show for the first time, to our knowledge, that ASCC1 inhibits NF-κB activation and that a truncated and inactive variant of ASCC1 is associated with a more severe disease, which could have clinical value for assessing the progression and prognosis of RA.