West Nile virus (WNV) is a pathogen to cause West Nile encephalitis when the infection occurs in the brain. Previous studies in mice identified the 2'-5' oligoadenylate synthetase 1b (Oas1b) gene as a determining factor for resistance to WNV infection. In addition, it has been suggested that human OAS1 and OASL are associated with the resistance to the WNV infection. WNV is maintained in nature through a complex life cycle involving wildbirds and mosquitoes. Birds are not only susceptible to the WNV, but also act as reservoir hosts, thus participating in the spread of the disease. It has previously been reported that chicken OASL possesses the oligoadenylate synthetase activity. However, until now the antiviral activity of chicken OASL has not been determined. In this study, we investigated the putative antiviral activity of chicken OASL by ectopic expression of this enzyme in mammalian cells and then infecting these cells with WNV replicon. We demonstrate that chicken OASL has an antiviral activity against the WNV. This is the first report to show that chicken OASL is associated with the resistance to the WNV infection.