During the screening of anti-influenza virus substances from traditional herbal medicines, the methanol extract from the leaves of Pogostemon cablin Benth. showed potent in vitro antiviral activity (99.8% inhibition at a concentration of 10 μg/mL) against influenza virus A/PR/8/34 (H1N1). The anti-influenza virus principle was isolated from the hexane-soluble fraction, through solvent fractionation, repeated silica gel column chromatography, and reversed-phase HPLC. The major active principle was a volatile substance that was identified as a sesquiterpene, patchouli alcohol (1), on the basis of its spectral analyses. When anti-influenza virus activity against A/PR/8/34 was evaluated by the plaque forming assay, patchouli alcohol reduced the number of plaques by 75% at 2 μg/mL and 89% at 10 μg/mL. Patchouli alcohol showed dose-dependent anti-influenza virus activity, and its IC(50) value was estimated to be 2.635 μM. Although 11 different sesquiterpenes were tested for antiviral activity against influenza virus A/PR/8/34, no or negligible activity was observed except for patchouli alcohol. Patchouli alcohol did not show anti-influenza virus activity against A/Guizhou/54/89 (H3N2), but showed weak activity against B/Ibaraki/2/85 (IC(50) = 40.82 μM). Patchouli alcohol did not show inhibitory activity against influenza virus neuraminidase.