The extract of Eriodictyon californicum has been widely used as an essential emollient, and as an anti-inflammatory, antibacterial, and antifungal agent. In this study, antitumor effects of E. californicum were evaluated against HCT116 human colon cancer cells. Treatment with the ethyl acetate extract of E. californicum (EEC) inhibited the short-term viability and long-term clonogenicity of HCT116 cells. We identified several flavonoids, including rosmarinic acid and luteolin, from the EEC by using high-performance liquid chromatography and nuclear magnetic resonance. Luteolin inhibited clonogenicity, activated caspase-7, and induced apoptosis of HCT116 cells. These findings suggest that EEC contains bioactive flavonoids that exhibit antitumor activities.