Numerous breast cancer patients experience cognitive changes during and after chemotherapy. Chemotherapy-related cognitive impairment can significantly affect quality of life. This pilot study attempted to determine the effects of a compensatory cognitive training on the objective and subjective cognitive functioning of breast cancer patients receiving adjuvant chemotherapy. Fifty-four patients were assigned to either a compensatory cognitive training or waitlist condition. They were assessed at baseline (T1), the completion of the 12-week intervention (T2), and 6 months after intervention completion (T3). Outcomes were assessed using the standardized neuropsychological tests and the Functional Assessment of Cancer Therapy-Cognitive Function (FACT-Cog), version 3. Raw data were converted to T-scores based on baseline scores, and a repeated-measures ANCOVA, adjusting for age, intelligence, depression, and treatment, was used for analysis. The effect sizes for differences in means were calculated. The intervention group improved significantly over time compared to the waitlist group on objective cognitive function. Among ten individual neuropsychological measures, immediate memory, delayed memory, verbal fluency in category, and verbal fluency in letter showed significant group x time interaction. In subjective cognitive function, scores of the waitlist group significantly decrease over time on perceived cognitive impairments, in contrast to those of the intervention group. The 12-week compensatory cognitive training significantly improved the objective and subjective cognitive functioning of breast cancer patients. Because this was a pilot study, further research using a larger sample and longer follow-up durations is necessary.