Background: Galla Rhois has been considered to have medicinal properties against diarrhea, excessive sweating, bleeding, and chronic cough in Asian countries. Gallotannins, which are Galla Rhois-derived tannins, have been reported to possess biological and pharmacological activities, especially anticancer activity. In this study, we evaluated the effect of steaming at a temperature over 120 °C on the chemical constituents and biological activities of the water extract of Galla Rhois (GRE). Methods: GRE was steamed at a temperature over 120 °C (AGRE), and its specific constituents were analyzed; the results were validated using a high-performance liquid chromatography–diode array detector system. To evaluate the anticancer effect of GRE and AGRE, cell viability assay, cell cycle analysis, and Western blot analysis were performed in HCT116 human colon cancer cells. Results: Steaming markedly increased the contents of gallic acid and ellagic acid in GRE, and GRE or AGRE treatment reduced the viability of HCT116 cells. Notably, the steaming process enhanced the growth inhibitory effect of GRE in cancer cells. AGRE induced apoptosis through the activation of caspase-3, caspase-8, and caspase-9. Additionally, AGRE regulated the activation of mitogen-activated protein kinases including extracellular signal-regulated kinase, p38, and c-Jun NH2-terminal kinase, whereas GRE did not. However, both GRE and AGRE inhibited the activation of AKT. Conclusion: Compared with GRE, AGRE is more potent in its ability to induce apoptosis in HCT116 cells; therefore, we suggest that the steaming process may be useful as a feasible method for improving the anticancer effect of GRE.