ABSTRACT The standardized 80% ethanolic extract of the leaves of Ocimum basilicum var. thyrsiflorum (L.) Benth., Lamiaceae, growing in KSA, exhibited a significant antioxidant activity compared to the ethyl acetate and butanol extracts, which was correlated to its higher phenolic and flavonoid contents. Chromatographic separation of the 80% ethanol extract resulted in the isolation of ten known compounds; cinnamic acid, gallic acid, methylgallate, ellagic acid, methyl ellagic acid, apigenin, luteolin, vitexin, isovitexin, and 3"-O-acetylvitexin. Compound 3"-O-acetylvitexin, a C-glycosylated derivative of apigenin, was isolated for the first time from genus Ocimum. The 80% ethanolic extract and 3"-O-acetylvitexin showed significant cytotoxic activities against the HCT116 human colon cancer cell line [IC50 values 22.3 ± 1.1 and 16.8 ± 2.0 µg/ml (35.4 µM), respectively].