Background and purpose Sleep disorder is one of the most common problems during menopause, which affects different areas of people's lives. Today, mindfulness is one of the concepts that have attracted a lot of attention due to its desirable effects and low side effects. The present study was conducted to investigate the effect of mindfulness-based stress reduction (MBSR) group training on sleep quality of postmenopausal women. Method This is a quasi-experimental parallel study. The study involved 66 eligible postmenopausal women 45–60 years’ old (n = 33 in each group) during 2019–2020. The minimum score of women was 5 from Pittsburgh questionnaire. Eight sessions weekly (120-min/session) MBSR group training was conducted in the intervention group and menopausal health in the control group for two months. Pittsburgh Sleep Quality Questionnaire was used as a data collection tool. Descriptive statistics and nonparametric tests were utilized to analyze the data using SPSS software (version 25). The significance level < 0.05 was considered. Results Socio-demographic characteristics of participants were no statistically significant difference between the study groups before the intervention. The results showed that mean of overall quality of sleep score was before the intervention 10.21 ± 3.03, after the intervention 4.7 ± 2.45, and one month after the intervention 4.69 ± 2.4 respectively in intervention group (P < 0.001). While there was no significant change in the mean overall quality of sleep quality in the control group. Conclusion According to the results, MBSR group training is an effective strategy for improving the sleep quality of postmenopausal women. Therefore, could be used to improve the sleep quality of postmenopausal women by midwifery consultants in health centers.