<jats:p>Colorectal cancer is a growing health concern with increasing mortality rates, and resistance to anticancer drugs and radiotherapy is a serious drawback in its treatment. Auraptene is a natural prenyloxycoumarin with valuable anticancer effects. The aim of current study was to determine the synergy between auraptene, ionizing radiation, and chemotherapeutic drugs in colon adenocarcinoma cells for the first time. To do so, HT29 cells were treated with combination of auraptene + cisplatin, + doxorubicin, or + vincristine. Furthermore, cells were pretreated with nontoxic auraptene and then exposed to various doses of X‐radiation. Assessment of cell viability not only indicated significant (<jats:italic>p</jats:italic> < 0.05) synergic effects of auraptene and anticancer agents, also revealed more significant (<jats:italic>p</jats:italic> < 0.01) increase in the toxicity of applied radiations in auraptene pretreated cells. Interesting synergy between auraptene and radiotherapy was then confirmed by morphological alterations, DAPI staining, and flow cytometric analysis of the cell cycle. Moreover, real‐time reverse transcription polymerase chain reaction analysis indicated significant (<jats:italic>p</jats:italic> < 0.01) overexpression of p21<jats:italic>,</jats:italic> but not GATA6, in auraptene pretreated cells after radiotherapy, and also significant (<jats:italic>p</jats:italic> < 0.01) down regulation of CD44 and ALDH1 by auraptene. According to present results, auraptene could be considered as an effective natural coumarin to improve the outcome of current chemoradiotherapy options. Copyright © 2017 John Wiley & Sons, Ltd.</jats:p>