In the absence of efficient preventive vaccines, topical microbicides offer an attractive alternative in the prevention of Herpes simplex type 1 (HSV-1) and type 2 (HSV-2) infections. Because of their recognized anti-adhesive activity against bacterial pathogens, cranberry (Vaccinium macrocarpon Ait.) extracts may represent a natural source of new antiviral microbicides. However, few studies have addressed the applications of cranberry extract as a direct-acting antiviral agent. Here, we report on the ability of the novel cranberry extract Oximacro and its purified A-type proanthocyanidins (PACs-A), to inhibit HSV-1 and HSV-2 replication in vitro. Analysis of the mode of action revealed that Oximacro prevents adsorption of HSV-1 and HSV-2 to target cells. Further mechanistic studies confirmed that Oximacro and its PACs-A target the viral envelope glycoproteins gD and gB, thus resulting in the loss of infectivity of HSV particles. Moreover, Oximacro completely retained its anti-HSV activity even at acidic pHs (3.0 and 4.0) and in the presence of 10% human serum proteins; conditions that mimic the physiological properties of the vagina - a potential therapeutic location for Oximacro. Taken together, these findings indicate Oximacro as an attractive candidate for the development of novel microbicides of natural origin for the prevention of HSV infections.