Viral infection activates the transcription factors IRF3 and NF-κB, which synergistically induce type I interferons (IFNs). Here, we identify the E3 ubiquitin ligase RNF138 as an important negative regulator of virus-triggered IRF3 activation and IFN-β induction. Overexpression of RNF138 inhibited virus-induced activation of IRF3 and transcription of the IFNB1 gene, whereas knockout of RNF138 promoted virus-induced activation of IRF3, and transcription of the IFNB1 gene. We further found that RNF138 promotes ubiquitination of PTEN and subsequently inhibits PTEN interactions with IRF3, which is essential for PTEN-mediated nuclear translocation of IRF3, thereby inhibiting IRF3 import into the nucleus. Our findings suggest that RNF138 negatively regulates virus-triggered signaling by inhibiting the interaction of PTEN with IRF3, and these data provide new insights into the molecular mechanisms of cellular antiviral responses.