Background: Rhein is a natural agent isolated from the traditional Chinese medicine rhubarb, which has been used as a medicine in China since ancient times. Although rhein was found to have significant anticancer effects in different cancer models, the effect and the underlying mechanisms of action of rhein in colorectal cancer (CRC) remain unclear. The mTOR/p70S6 kinase (p70S6K) pathway has been demonstrated as an attractive target for developing novel cancer therapeutics.Methods: The human CRC cell lines HCT116, HCT15, and DLD1 and xenograft mice were used in this study to investigate the effects of rhein. Assessments of cellular morphology, cell proliferation, and anchorage-independent colony formation were performed to examine the effects of rhein on cell growth. Wound healing assay and transwell migration and invasion assay were conducted to detect cell migration and invasion. Cell cycle and apoptosis were investigated by flow cytometry and verified by immunoblotting. Tissue microarray was used to detect mTOR expression in patients with CRC. Gene overexpression and knockdown were implemented to analyze the function of mTOR in CRC. The in vivo effect of rhein was assessed in a xenograft mouse model.Results: Rhein significantly inhibited CRC cell growth by inducing S phase cell cycle arrest and apoptosis. It also inhibited CRC cell migration and invasion ability through EMT process. mTOR was highly expression in CRC cancer tissues and cells exhibited high mTOR expression. Overexpression of mTOR promoted cell growth, migration, and invasion ability, whereas mTOR knockdown diminished these phenomena of CRC cells in vitro. Moreover, rhein directly targeted mTOR and suppressed the mTOR/p70S6K signaling pathway in CRC cells. Intraperitoneal administration of rhein inhibited CRC cell HCT116 xenograft tumor growth through the mTOR/p70S6K pathway.Conclusions: Rhein exerted anticancer activity in vitro and in vivo through directly targeting mTOR and inhibiting mTOR/p70S6K signaling pathway. These data indicate that rhein is a potent anticancer agent that could be useful for the prevention or treatment of CRC.