Bunyaviruses pose a significant threat to human health, prosperity and food security. In response to viral infections, interferons (IFNs) upregulate the expression of hundreds of interferon stimulated genes (ISGs) whose cumulative action can potently inhibit the replication of bunyaviruses. We used a flow cytometry-based method to screen the ability of ?500 unique ISGs from humans and rhesus macaques to inhibit the replication of Bunyamwera orthobunyavirus (BUNV), the prototype of both the Peribunyaviridae family and Bunyavirales order. Candidates possessing antibunyaviral activity were further examined using a panel of divergent bunyaviruses. Interestingly, one candidate, ISG20, exhibited potent antibunyaviral activity against most viruses examined from the Peribunyaviridae, Hantaviridae and Nairoviridae families, whereas phleboviruses (Phenuiviridae) largely escaped inhibition. Similar to other viruses known to be targeted by ISG20, the antibunyaviral activity of ISG20 is dependent upon its functional ribonuclease activity. Through use of an infectious VLP assay (based on the BUNV minigenome system), we confirmed that gene expression from all 3 viral segments is strongly inhibited by ISG20. Using in vitro evolution, we generated a substantially ISG20-resistant BUNV and mapped the determinants of ISG20 sensitivity/resistance. Taken together, we report that ISG20 is a broad and potent antibunyaviral factor yet some bunyaviruses are remarkably ISG20 resistant. Thus, ISG20 sensitivity/resistance could influence the pathogenesis of bunyaviruses, many of which are emerging viruses of clinical or veterinary significance.IMPORTANCE There are hundreds of bunyaviruses and many cause life-threatening acute diseases in humans and livestock. The interferon (IFN) system is a key component of innate immunity and type I IFNs limit bunyaviral propagation both in vitro and in vivo. Type I IFN signalling results in the upregulation of hundreds of IFN stimulated genes (ISGs) whose concerted action generates an antiviral state. Although IFNs are critical in limiting bunyaviral replication and pathogenesis, much is still unknown about which ISGs inhibit bunyaviruses. Using ISG expression screening, we examined the ability of ?500 unique ISGs to inhibit Bunyamwera orthobunyavirus (BUNV), the prototypical bunyavirus. Using this approach we identified ISG20, an interferon stimulated exonuclease, as a potent inhibitor of BUNV. Interestingly ISG20 possesses highly selective antibunyaviral activity and multiple bunyaviruses are potently inhibited whilst some largely escape inhibition. We speculate that the ability of some bunyaviruses to escape ISG20 could influence their pathogenesis.