The polyphenol-enriched aqueous extract RF from the aerial parts of Rhododendron ferrugineum exhibited strong antiviral activity against herpes simplex virus type-1 while adenovirus 3 was not affected. RF exhibited an IC(50) of 7.4 μg/mL and a selectivity index of 64 when added to the virus inoculum prior to infection. RF abolished virus entry into the host cell by blocking attachment to the cell surface. When added after attachment at a concentration of >25 μg/mL, RF inhibited also penetration of HSV-1 into the host cell. RF directly interacts with viral envelope proteins as demonstrated for the viral glycoprotein gD.