Irinotecan, an analog of camptothecin, which is an inhibitor of topoisomerase I, is currently used in the treatment of metastatic colorectal cancer. Camptothecin derivatives have been demonstrated to exert radiosensitizing effects on several types of cancer cells. However, to date, at least to the best of our knowledge, few studies have examined these effects in colorectal cancer cell lines. In the present study, we examined the radiosensitizing effects of irinotecan on the p53-mutant colorectal cancer cell lines, HT29 and SW620, and explored the potential underlying mechanisms. Drug cytotoxicity tests revealed that the 24 h half-maximal inhibitory concentrations (IC50s) of irinotecan as a single agent were 39.84 µg/ml (HT29 95% CI, 38.27-41.48) and 96.86 µg/ml (SW620 95% CI, 89.04-105.4); finally, concentrations <2 µg/ml were used in the subsequent experiments. Clonogenic assays revealed that irinotecan exerted radiosensitizing effects on the HT29 and SW620 cells, and the sensitivity enhancement ratios (SERs) at 2 Gy increased with increasing concentrations (SER at 2 Gy, 1.41 for the HT29 cells, 1.87 for the SW620 cells; with irinotecan at 2 µg/ml). Subsequently, the cells were divided into 4 groups: The control group, irinotecan group, radiation group and combination group. Compared with the control, irinotecan and radiation groups, the combination group had the slowest cell growth rate and the most obvious foci of Ser139p‑γH2AX. Combined treatment resulted in a firstly decreased and then increased M phase arrest and led to the most significant G2/M phase arrest, followed by the most significant increase in apoptosis. The results of western blot analysis indicated that the expression levels of proteins related to the DNA damage response system (Ser1981p‑ATM, Ser345p‑Chk1, Thr68p‑Chk2 and Ser139p‑γH2AX) and the cell cycle (Tyr15p‑Cdc2 and cyclin B1) exhibited the greatest increase in the combined group. In addition, the expression of Ser216p‑Cdc25C was also increased in the combined group, indicating that irinotecan likely radiosensitized the p53-mutant HT29 and SW620 cells through the ATM/Chk/Cdc25C/Cdc2 pathway.