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Galactic cosmic radiation (GCR) is an unavoidable risk to astronauts that may affect mission success. Male rodents exposed to 33‐beam‐GCR (33‐GCR) show short‐term cognitive deficits but reports on female rodents and long‐term assessment are lacking. We asked: What are the longitudinal behavioral effects of 33‐GCR on female mice? Also, can an antioxidant/anti‐inflammatory compound (CDDO‐EA) mitigate the impact of 33‐GCR? Mature (6‐month‐old) C57BL/6J female mice received CDDO‐EA (400 μg/g of food) or a control diet (vehicle, Veh) for 5 days and Sham‐irradiation (IRR) or whole‐body 33‐GCR (0.75Gy) on the 4th day. Three‐months post‐IRR, mice underwent two touchscreen‐platform tests: (1) location discrimination reversal (tests behavior pattern separation and cognitive flexibility, abilities reliant on the dentate gyrus) and (2) stimulus–response learning/extinction. Mice then underwent arena‐based behavior tests (e.g. open field, 3‐chamber social interaction). At the experiment's end (14.25‐month post‐IRR), an index relevant to neurogenesis was quantified (doublecortin‐immunoreactive [DCX+] dentate gyrus immature neurons). Female mice exposed to Veh/Sham vs. Veh/33‐GCR had similar pattern separation (% correct to 1st reversal). There were two effects of diet: CDDO‐EA/Sham and CDDO‐EA/33‐GCR mice had better pattern separation vs. their respective control groups (Veh/Sham, Veh/33‐GCR), and CDDO‐EA/33‐GCR mice had better cognitive flexibility (reversal number) vs. Veh/33‐GCR mice. One radiation effect/CDDO‐EA countereffect also emerged: Veh/33‐GCR mice had slower stimulus–response learning (days to completion) vs. all other groups, including CDDO‐EA/33‐GCR mice. In general, all mice showed normal anxiety‐like behavior, exploration, and habituation to novel environments. There was also a change relevant to neurogenesis: Veh/33‐GCR mice had fewer DCX+ dentate gyrus immature neurons vs. Veh/Sham mice. Our study implies space radiation is a risk to a female crew's longitudinal mission‐relevant cognitive processes and CDDO‐EA is a potential dietary countermeasure for space‐radiation CNS risks. [ABSTRACT FROM AUTHOR]