Context • Breast cancer is a leading cause of cancer fatalities among women worldwide. Of the more than 80% of patients who receive adjuvant chemotherapy, approximately 40% relapse. The majority of these patients die of disseminated metastatic disease, which emphasizes the need for new therapeutic strategies Objective • The study intended to investigate the anticancer effects of oleuropein (OL) and doxorubicin (DOX) individually and in combination on breast tumor xenografts and also to evaluate the molecular pathways involved. Design • The research team designed in vivo (animal) and in vitro (cell culture) studies. Setting: The study was performed in the College of Science of King Saud University in the University Center for Women Students (Riyadh, Saudi Arabia). Animals • The study involved 40 female, nude mice (BALB/c OlaHsd-foxn1). Intervention • The mice were injected subcutaneously with MDA-MB-231 human breast cancer cells. After the growth of tumors, the animals were randomly divided into 4 groups to receive intraperitoneal injections: (1) group 1 (control group)—dimethyl sulfoxide, (2) group 2 (intervention group)—50 mg/kg of OL, (3) group 3 (intervention group)—2.5 mg/kg of DOX, and (4) group 4 (intervention group)—1.5 mg/kg of DOX, immediately followed by 50 mg/kg of OL. The OL was extracted from Manzanillo olive trees (Olea europaea) grown in Tabouk, Saudi Arabia. Outcome Measures • The measures included the isolation and primary culture of the tumor xenografts, apoptosis analysis by annexin V, cellular lysate preparation, and immunoblotting. Results: The volume of the tumor increased aggressively, reaching 173 mm3 in the control animals in a time-dependent manner. On the other hand, a sharp drop, to 48.7 mm3, in the volume of the tumor was observed with the 2 drugs combined, a more than 3-fold decrease. The effect was mediated through the induction of apoptosis via the mitochondrial pathway. The combined treatment downregulated the antiapoptosis and proproliferation protein, nuclear factor-kappa ?, and its main oncogenic target cyclin D1. Furthermore, it inhibited the expression of BCL-2 and survivin. This inhibition could explain the cooperative suppression of the proliferation of breast tumor xenografts and the induction of apoptosis by the combined effect of the compounds used. Conclusions • The key findings clearly indicate the synergistic efficacy of DOX with natural and nontoxic OL against breast tumor xenografts.