The DNA sensing pathway triggers innate immune responses against DNA virus infection, and NF-B signaling plays a critical role in establishing innate immunity. We report here that the herpes simplex virus 1 (HSV-1) ubiquitinspecific protease (UL36USP), which is a deubiquitinase (DUB), antagonizes NF-B activation, depending on its DUB activity. In this study, ectopically expressed UL36USP blocked promoter activation of beta interferon (IFN-) and NF-B induced by cyclic GMP-AMP synthase (cGAS) and stimulator of interferon genes (STING). UL36USP restricted NF-B activation mediated by overexpression of STING, TANKbinding kinase 1, IB kinase ␣ (IKK␣), and IKK, but not p65. UL36USP was also shown to inhibit IFN-stimulatory DNA-induced IFN- and NF-B activation under conditions of HSV-1 infection. Furthermore, UL36USP was demonstrated to deubiquitinate IB␣ and restrict its degradation and, finally, abrogate NF-B activation. More importantly, the recombinant HSV-1 lacking UL36USP DUB activity, denoted as C40A mutant HSV-1, failed to cleave polyubiquitin chains on IB␣. For the first time, UL36USP was shown to dampen NF-B activation in the DNA sensing signal pathway to evade host antiviral innate immunity.IMPORTANCE It has been reported that double-stranded-DNA-mediated NF-B activation is critical for host antiviral responses. Viruses have established various strategies to evade the innate immune system. The N terminus of the HSV-1 UL36 geneencoded protein contains the DUB domain and is conserved across all herpesviruses. This study demonstrates that UL36USP abrogates NF-B activation by cleaving polyubiquitin chains from IB␣ and therefore restricts proteasome-dependent degradation of IB␣ and that DUB activity is indispensable for this process. This study expands our understanding of the mechanisms utilized by HSV-1 to evade the host antiviral innate immune defense induced by NF-B signaling.KEYWORDS HSV-1, DNA sensor, UL36, NF-B, IB␣ C ells have developed plenty of ways to govern their homeostasis, and the recognition of double-stranded DNA (dsDNA) by DNA sensors is a central host cellular defense against DNA virus infection. Among these DNA sensors, cyclic GMP-AMP synthase (cGAS), which is a nucleotidyltransferase, has been demonstrated to be the predominant cytosolic DNA sensor. Upon binding to DNA fragments, cGAS utilizes GTP and ATP to produce cyclic-GMP-AMP (cGAMP) through its enzymatic activity, and cGAMP activates an endoplasmic reticulum (ER)-resident receptor, stimulator of interferon genes (STING) (1). Activated STING then recruits TANK-binding kinase 1 (TBK1) and traffics from the ER to a perinuclear endosomal compartment, leading to the activation of transcription factors NF-B and interferon (IFN) regulatory factor 3 (IRF3), which induce IFN- production (1-4).