Viral hemorrhagic septicemia causes significant economic losses for olive flounder (Paralichthys olivaceus) farms in Korea. The purpose of this study was to optimize extraction conditions for licorice (Glycyrrhiza uralensis roots) extract using the 23 full factorial design and thereby optimizing the antiviral activity against viral hemorrhagic septicemia virus (VHSV). The optimum conditions were ethanol concentration 100%, extraction temperature 40˚C and extraction time 5 h. Optimized extract (OE) prepared under these conditions showed potent antiviral activity against VHSV with a 95.2% CPE reduction at 75 µg/ml of OE, and a predictive capacity of 94%. Fish that received 250 and 500 mg/kg body weight/day of OE as either preventive or therapeutic administration had higher relative survival rates of 25 to 62.5% (p < 0.05) compared with VHSV‐infected control group. Active components of licorice, glycyrrhizin (GL) and glycyrrhetinic acid (GLA) showed antiviral activity against VHSV with 50% effective concentration values of 100.6 and 26.0 µM respectively. Besides, time course studies by plaque reduction assay showed that GLA exhibited a direct virucidal effect on VHSV or may inhibit the penetration and internalization of the virus to cells. These results suggest that OE may be used as an anti‐VHSV agent in aquaculture.