Background Change in cognitive ability is a commonly reported adverse effect by breast cancer survivors. The underlying etiology of cognitive complaints is unclear and to date, there is limited evidence for effective intervention strategies. Exercise has been shown to improve cognitive function in older adults and animal models treated with chemotherapy. This proof‐of‐concept randomized controlled trial tested the effect of aerobic exercise versus usual lifestyle on cognitive function in postmenopausal breast cancer survivors. Methods Women, aged 40 to 65 years, postmenopausal, stages I to IIIA breast cancer, and who self‐reported cognitive dysfunction following chemotherapy treatment, were recruited and randomized to a 24‐week aerobic exercise intervention (EX; n = 10) or usual lifestyle control (CON; n = 9). Participants completed self‐report measures of the impact of cognitive issues on quality of life (Functional Assessment of Cancer Therapy–Cognitive version 3), objective neuropsychological testing, and functional magnetic resonance imaging at baseline and 24 weeks. Results Compared to CON, EX had a reduced time to complete a processing speed test (trail making test‐A) (‐14.2 seconds, P < .01; effect size 0.35). Compared to CON, there was no improvement in self‐reported cognitive function and effect sizes were small. Interestingly, lack of between‐group differences in Stroop behavioral performance was accompanied by functional changes in several brain regions of interest in EX compared to CON at 24 weeks. Conclusion These findings provide preliminary proof‐of‐concept results for the potential of aerobic exercise to improve cancer‐related cognitive impairment and will serve to inform the development of future trials.