Background/Aim: Colorectal cancer (CRC) is the third most common cancer with a high mortality rate worldwide. Although gallic acid and hesperidin exert anticancer activity, synergistic effects of gallic acid and hesperidin against CRC remain elusive. This study aims to investigate the therapeutic mechanism of a novel combination of gallic acid and hesperidin against CRC cell growth, including cell viability, cell-cycle-associated proteins, spheroid formation, and stemness. Methods: Gallic acid and hesperidin derived from Hakka pomelo tea (HPT) were detected by colorimetric methods and high-performance liquid chromatography using ethyl acetate as an extraction medium. CRC cell lines (HT-29 and HCT-116) treated with the combined extract were investigated in our study for cell viability (trypan blue or soft agar colony formation assay), cell cycle (propidium iodide staining), cell-cycle-associated proteins (immunoblotting), and stem cell markers (immunohistochemistry staining). Results: Compared with other extraction methods, HPT extraction using an ethyl acetate medium exerts the most potent effect on inhibiting HT-29 cell growth in a dose-dependent manner. Furthermore, the treatment with combined extract had a higher inhibitory effect on CRC cell viability than gallic acid or hesperidin alone. The underlying mechanism was involved in G1-phase arrest and Cip1/p21 upregulation that could attenuate HCT-116 cell proliferation (Ki-67), stemness (CD-133), and spheroid growth in a 3D formation assay mimicking in vivo tumorigenesis. Conclusion: Gallic acid and hesperidin exert synergistic effects on cell growth, spheroids, and stemness of CRC and may serve as a potential chemopreventive agent. Further testing for the safety and effectiveness of the combined extract in large-scale randomized trials is required.