Standard cancer therapy targets tumor cells without considering the possible collateral damage on the tumor microenvironment that could impair therapy response. Employing patient-derived tumor organoids and primary stroma cells or a novel murine rectal cancer model, we show that interleukin-1a (IL-1a) dependent inflammatory cancer-associated fibroblast (iCAF) polarization triggers oxidative DNA damage in iCAFs leading to p53-mediated therapy-induced senescence associated with changes in matrisome composition, chemoradiotherapy resistance and disease progression. IL-1 inhibition, prevention of iCAF senescence or senolytic therapy sensitizes mice to irradiation. In rectal cancer patients a dominant iCAF gene signature as well as lower IL-1 receptor antagonist (IL-1RA) serum levels correlate with poor prognosis. Moreover, conditioned supernatant from patient tumor organoids renders fibroblasts susceptible to radiation-induced senescence in an IL-1-dependent manner. Collectively, we unravel a critical role for iCAFs in therapy resistance and identify IL-1 signaling as an attractive target for stroma-repolarization and prevention of CAF senescence.