To investigate the antiviral activity of marine algae against fish pathogenic viruses, which are often the causes of viral disease in aquaculture, the 80% methanolic extracts of 21 species collected from the coast of Korea were screened for their in vitro antiviral activities on infectious hematopoietic necrosis virus (IHNV) and infectious pancreatic necrosis virus (IPNV), using a flounder spleen (FSP) cell-line. Among them, Monostroma nitidum (10 ${mu}g/mL$) exhibited the strongest inactivation on IHNV, showing a 2 log reduced virus titre as compared to the control in the determination of direct virucidal activity. In addition, Polysiphonia morrowii (100 ${mu}g/mL$) remarkably reduced the virus titres of treated cells by 2-2.5 log, for both IHNV and IPNV, in the determination of cellular protective activity, implying the existence of substances that may modulate innate host defense mechanisms against viral infections. These results reveal that some marine algae could be promising candidates as sources of antiviral agents or as health-promoting feeds for aquaculture.