The hot water extract of a brown marine alga, Cystoseira myrica, from the Persian Gulf was evaluated as an antiviral compound against KOS strain of HSV-1 in cell culture. The extract exhibited antiviral activity against herpes simplex virus type 1 (HSV-1) not only during absorption of virus to the cells, but also on post attachment stages of virus replication. The water extract of C. myrica was sterilized by filtration and autoclaving, respectively. The IC 50 for filtered extract was 99 µg/ml and the IC 50 for autoclaved extract was 125 µg/ml. Based on resulted selectivity index (SI) values of the extracts, which were 33.4 and 28.2 for filtered and autoclaved extracts, respectively, we found that the antiviral compound(s) in the water extract of C. myrica to be heat stable. Also, the SI values for inhibition of the post attachment stages of HSV-1 replication were 23.1 and 21.7 for filtered and autoclaved extracts, respectively. The IC 50 in this phase of study were 143 and 162 µg/ml for filtered and autoclaved extracts, respectively. Therefore, C. myrica could be a good candidate as a natural source for anti-HSV-1 compound(s) isolation.