Efficient and productive virus infection often requires viral countermeasures that block innate immunity. The interferon-inducible 2′,5′-oligoadenylate (2-5A) synthetases (OAS) and ribonuclease L (RNase L) are components of a potent host antiviral pathway. We previously showed that murine coronavirus (MHV) accessory protein ns2, group A rotavirus (RVA) VP3 carboxy-terminal domain (VP3-CTD), and mammalian AKAP7 are members of 2H phosphoesterase superfamily with 2′-phosphodiesterase (2′-PDE) activity that potently cleaves 2-5A thereby preventing activation of RNase L (Zhao, Jha et al., PMID: 22704621; Zhang, Jha et al., PMID: 23878220 and Gusho, Zhang, Jha et al., PMID: 24987090). Here, we will demonstrate that Middle East respiratory syndrome (MERS)-CoV gene NS4b encodes a homologous and similar PDE that cleaves 2-5A in vitro (km/Kcat = 12.1 M⁻¹ s⁻¹), inhibits 2-5A accumulation in cell culture and prevents ribosomal (r) RNA degradation in murine bone marrow macrophages (BMM), a hallmark of RNase L antagonism, and rescues an MHV mutant virus with a catalytically inactive NS2a protein unable to antagonize RNase L in vivo. Interestingly, NS4b has a nuclear localization signal however there is a mixed nuclear/cytoplasmic localization when overexpressed in human airway cell line A549 and in BMM when expressed from chimeric MHV. Viral evasion of OAS/RNase L pathway is a critical hepatovirulence determinant for lineage A Betacoronavirus mouse hepatitis virus (MHV). Taken together, our data suggest that RNase L antagonism may be a critical component of MERS-CoV pathogenesis. Additionally, this is the first evidence of RNase L antagonism by a lineage C Betacoronavirus.