Abstract In a previous study, we demonstrated long-term decreased intestinal epithelial cell migration after low-dose heavy ion iron radiation, and this was associated with increased senescence-associated secretory phenotype (SASP) signaling. However, we know very little about the long-term effects of low dose space radiation on intestinal stem cell (ISC) senescence and SASP that have implications for intestinal homeostasis. Male Lgr5+ (Lgr5-EGFP-IRES-creERT) mice or Lgr5+/APC1638N/+ were exposed to 0.5 Gy of sham, γ-rays, or 28Si (69 keV/μm) radiation and were euthanized 60 or 150 days after irradiation. ISCs were FACS sorted for oxidative stress analysis and assessed for senescence and SASP signaling in tissue sections by immunostaining. Greater cellROX or mitoSOX staining was observed after 28Si indicating higher oxidative stress compared to γ-rays or control Lgr5 positive cells. β-gal co-staining with Lgr5, lysozyme, IL1R, or ILβ showed highest staining in 28Si irradiated compared to controls ISCs samples. γ-H2AX staining showed greater foci numbers in 28Si relative to γ-rays or control group. Higher expression of secretory cytokines IL8 and IL15 was observed in 28Si irradiated ISCs relative to γ-rays or control group at both time points. Elevated expression of DKK2 and activation of NF-κB and p38 MAPK were observed in senescent stem cells of 28Si irradiated relative to γ-rays or control group at these time points. A higher GI tumor count was observed in 28Si compared to γ-irradiated or control Lgr5+/APC1638N/+ mice. In summary, our results demonstrate low dose heavy ion exposure induces ongoing DNA damage, persistent oxidative stress, senescence and acquisition of the SASP in ISCs. Higher levels of oxidative stress, senescence and SASP were observed at 150 compared to the 60-day timepoint. Overall, space radiation-induced oxidative stress, senescence, SASP, and activated p38MAPK/NF-κB pathway may very well have implications in GI carcinogenesis, accelerated aging, and pathological conditions, such as intestinal dysfunction, in astronauts during deep space flights. Since similar effects have been seen in our studies with other particle beams including carbon ions, this has potential relevance to particle radiotherapy as well. This work was supported by NASA NNX15AI21G Citation Format: Santosh Kumar, Shubhankar Suman, Kamal Datta, Albert J. Fornace. Heavy ion radiation exposure perturbs intestinal stem cell homeostasis and induces accelerated tumorigenesis in apc mutant mouse intestine [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3059.