Background. Apoptosis, a major form of programmed cell death, plays a vital role in regulating tissue development and maintenance of homeostasis in eukaryotes. Apoptosis can occur via a death receptor-dependent extrinsic or a mitochondrial-dependent intrinsic pathway and can be induced by various chemotherapeutic agents. In this study, the anticancer activity of Saussurea costus and its mode of intervention in human cancer cells of breast, colon, and liver were investigated. Results. In this study, the bioactives of S. costus leaves were extensively extracted in five solvents of different polarity. The cytotoxicity and anticancer effect of the extracted secondary metabolites were investigated against breast (MCF-7), liver (HepG2), and colon (HCT116) cancer cell lines using a Sulphorhodamine B (SRB) assay. Secondary metabolites extracted using hexane, methanol, ethyl acetate, and chloroform had the highest cytotoxicity and thus the greatest anticancer effect on all the cancer cell lines tested (IC₅₀; ranging from 0.25 to 2.5 μg/ml), while butanol was comparatively less active (IC₅₀; ranging from 23.2 to 25.5 μg/ml). Further investigation using DNA flow cytometry and fluorescent microscopy revealed that the extract arrested the cells in the G₁ phase of cell cycle and induced apoptosis. Furthermore, the elevated expression level of proapoptotic proteins and decreased expression level of antiapoptotic proteins confirmed that the intrinsic (mitochondrial) pathway was involved in mediating the apoptosis of cancer cells upon treatment with S. costus extract. These results altogether suggest that S. costus could be a potential anticancer agent. Conclusion. These results suggest that the S. costus extract is the potential source of the secondary metabolites that could be used as anticancer agent to treat diverse cancers of breast, colon, and liver.