A new indole alkaloid 1, together with four known indole alkaloids 2-5, was isolated from the 95% EtOH extract of the traditional Chinese medicinal plant Isatis indigotica. Alkaloids 2-5 were found in the plant for the first time. Their structures were elucidated as 3-hydroxy-3-acetonitrile-4-hydroxy-2-indolone (1), 1-methoxy-3-indoleacetonitrile (2), 3-indoleacetic acid (3), 3-indolealdehyde (4), and 1-methoxy-3-indolecarbaldehyde (5) on the basis of spectroscopic data. Their anti-HSV effects in vitro were evaluated by plaque reduction assay.