Orthosiphon pallidus (O. pallidus), which belongs to the Lamiaceae family, is a popular garden plant that is widely used for the treatment of various diseases, such as urinary lithiasis, fever, hepatitis, cancer and jaundice. The objective of the present work was to investigate the antioxidant free-radical scavenging and the anticancer activities of O. pallidus against human breast-cancer cell lines.The antioxidant activity of Orthosiphon pallidus aqueous extract (OPAE) was investigated using different models, such as the 1,1-diphenyl-2-picrylhydrazyl (DPPH) and the 2, 2'-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) models, as were the Fe+ chelation, the hydroxyl radical and superoxide radical scavenging, and total reducing power activities. The anticancer activities of the extract were determined by using the 3-(4, 5- dimethylthiazolyl-2)-2, 5-diphenyltetrazolium bromide (MTT) and the sulforhodamine (SRB) assays on the MCF-7 and the MDA-MB-231 cancer cell lines.The aqueous Orthosiphon pallidus extract showed potent activity in in-vitro models. It significantly inhibited the scavenging of hydroxyl and superoxide radicals, but induced a remarkable Fe+ chelation activity. For both cell lines, the percent cytotoxicity was found to increase steadily with increasing OPAE concentration up to 240 μg/mL.These results suggest that Orthosiphon pallidus has excellent antioxidant, antimicrobial, and anticancer activities against human breast-cancer cell lines.