Purpose This study examined the efficacy of a brief acceptance and commitment therapy (ACT) on subjective cognitive impairment in breast cancer patients undergoing chemotherapy. Methods Data collection was carried out in 3-time points: baseline (T1), screening (T2), and post-treatment (T3). Respondents who had significant subjective cognitive impairment were randomly divided into two groups: intervention (n = 30) and waitlist (n = 30). Respondents in the intervention group received 4 sessions of 1 hour of ACT therapy. Findings Respondents in the intervention group showed significant improvement in subjective cognitive impairment, depression, anxiety, and psychological inflexibility after the ACT intervention (p < 0.05). After controlling the covariates, group differences in all variables were significant except for fatigue and psychological inflexibility has the highest effect size (d = 4.69). Conclusion ACT could be considered as an effective intervention to ameliorate subjective cognitive impairment, anxiety, depression, and psychological inflexibility in breast cancer patients undergoing chemotherapy. Implications for Psychosocial Providers This study highlights the importance of screening for subjective cognitive impairment in breast cancer patients undergoing chemotherapy and heightens their opportunity to receive proper management as earlier as possible.