Introduction: Herpes simplex virus type 1 (HSV-1) is an enveloped DNA virus in the Herpesviridae family which causes various infections globally. Acyclovir (ACV) is a choice treatment, and the widespread usage of ACV has lead to the emergence of HSV strains resistant to ACV. Therefore, the discovery of novel anti-HSV drugs deserves great effort. Oliveria decumbens is a traditional medicinal plant used in Iran for many purposes, particularly for gastrointestinal disorders and analgesia. There is no report about antiviral effects of this plant; so, this report was the first study to evaluate antiviral effects of Oliveria decumbens. The aim of this study was to investigate the in vitro antiviral activity of Oliveria decumbens flowers methanolic extract against HSV-1.
 
Materials & Methods: In this laboratorial study, methanolic extract of the dried powder was obtained through maceration method. Then, the solvent was removed by rotary evaporator. The cytotoxicity and antiviral activity were evaluated by MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay in Vero cell line and at different incubation times. Cell cultures were treated with the extract before, during, and after infection to study its antiviral activity. 
 
Findings: The cytotoxicity results revealed that cytotoxic concentration of the extract which reduced viable cells number by 50% (CC50) was above the range of 1500µg/ml. The findings also indicated that the plant extract had the most antiviral activity when it was mixed with the virus, and this mixture was used to infect the cells where it reached a high antiviral activity of 43.75percent.
 
Discussion & Conclusion: As demonsterated by our gains, the tested methanolic extract displayed a moderate antiviral activity against herpes simplex virus-1.