Cancer represent one of the most serious health problems and major causes of death around the world. Many anticancer drugs in clinical use today are natural products or derived from natural sources. Withania somnifera (L.) Dunal is a small shrub widely distributed in many parts of the world including Saudi Arabia. The antiproliferative activities of the methanolic extract of W. somnifera leaves collected from Faifa mountains, southwest Saudi Arabia against MCF-7, HCT116 and HepH2 cell lines were investigated. The extract showed a strong antiproliferative activity against all cell lines with IC50 values of 3.35, 2.19 and 1.89 μg/ml, respectively. Flow cytometry results showed that the extract arrested the cell cycle at S phase, and the increase in the caspase 3 activity suggested that the extract could induce cell apoptosis by a caspase mediated pathway. These results demonstrated that the methanolic extract of W. somnifera leaves collected from Faifa mountains has comparable strong antiproliferative activities to samples collected from different locations.