Mediator of IFN regulatory transcription factor 3 activation (MITA) is an important adaptor protein to mediate the induction of type I IFNs. In this study, we identified an alternatively spliced isoform of MITA lacking exon 7, termed MITA-related protein (MRP). MRP shares the N-terminal portion aa 1-253 with MITA but possesses a unique 30-aa sequence at the carboxyl terminal part, therefore lacking the conserved domains including TANK-binding kinase 1 (TBK1) and cyclic diguanylate binding domain. MRP is expressed in multiple tissues and distinct cell lines. Overexpression of MRP inhibited MITA-mediated activation of IFN-β promoter by sendai virus infection and cyclic diguanylate treatment but enhanced that in HSV-1 infection. Interestingly, MRP expression was reduced after Sendai virus infection but was upregulated after HSV-1 infection. Overexpression of MRP inhibited MITA-mediated induction of IFN-β via TBK1-IFN regulatory transcription factor 3 by disrupting the MITA-TBK1 interaction. However, NF-κB pathway was still activated by MRP, as MRP retained the ability to interact with inducible inhibitor of NF-κB (iκB) kinase. Thus, MRP acts as a dominant negative regulator of MITA-mediated induction of IFN production.