Rosemary extracts have exhibited potential cytostatic or cytotoxic effects in several cancer cell models but their bioactive compounds are yet to be discovered. In this work, the anticancer activity of a rosemary-leaf extract and its fractions were assayed to identify the phenolic compounds responsible for their antiproliferative/cytotoxic effects on a panel of human colon cancer cell lines. Bioguided fractionation of the rosemary-leaf extract was achieved by semi-preparative chromatography. The rosemary extract and the compounds in the fractions were characterized and quantified by HPLC-ESI-QTOF-MS. Cellular viability in the presence of these fractions and the whole extract was determined after 24 or 48 h incubations by using an MTT assay. Fractions containing diterpenes or triterpenes were the most active but not as much as the whole extract. In conclusion, carnosic acid, carnosol, 12-methoxycarnosic acid, taxodione, hinokione and betulinic acid were the putative candidates that contributed to the observed antiproliferative activity of rosemary in human colon cancer cells. Whether the effects of the extract and fractions are only cytostatic or cytotoxic needs to be elucidated. Nevertheless, the comparative antiproliferative study on the fractions and whole extract revealed potential synergistic effects between several components in the extract that may deserve further attention.