The current treatment strategies for metastatic breast cancer depend on the cancer subtype by palliating symptoms and prolonging life. However, triple-negative breast cancers have no targeted treatment available. Orthosiphon stamineus (O.s) is a traditional folk medicine plant used in South East Asia to treat many diseases. The aim of this study is to evaluate the anti-tumor activity of O.s extract formulation (ID: C5EOSEW5050ESA trademarked as NuvastaticTM) and its major component, rosmarinic acid in a breast in vivo tumor xenograft model. Human triple-negative breast cancer cells were transplanted into the mammary fat pad of 20 athymic nude mice. Fourteen days post-injection, mice were randomly assigned to four groups. C5EOSEW5050ESA (200 and 400 mg/kg/day) and rosmarinic acid (32 mg/kg/day) were administered orally. The body weight and tumor size were measured twice a week. Histopathological analyses of tumor tissues were conducted. Tumor necrosis and tumor growth were determined by hematoxylin and eosin staining. A significant reduction in tumor size and growth was found in all treatment groups. No significant difference between C5EOSEW5050ESA at 200 mg/kg and rosmarinic acid in the reduction of tumor size and necrosis. However, a more significant reduction was found in tumor growth and necrosis with 400 mg/kg of C5EOSEW5050ESA treatment as compared to other treatments. These results highlighted the anti-tumor activity of C5EOSEW5050ESA in reducing breast tumor growth in mice compared to the lower dose of C5EOSEW5050ESA and rosmarinic acid. This study provides valuable insights into using C5EOSEW5050ESA as a treatment to target triple-negative breast cancers in vivo