Background: Cancers cause not only physical and social impairments but also psychological stress responses such as rumination, which require more attention from researchers as well as clinicians.Aim: This study aimed to investigate the effect of mindfulness training on rumination in women with breast cancer.Method: This randomized controlled trial was performed on 46 women with breast cancer in Bushehr city (2018). Subjects were randomly assigned to two intervention and control groups. The intervention was 8 sessions of group mindfulness training. A rumination questionnaire was administered before the intervention, immediately, and one month after the intervention. The data were analyzed by analytical tests at the 0.05 significance level.Results: There was no significant difference between the rumination scores of the intervention group at the three stages of measurement. For the control group, the mean rumination scores at the posttest and follow-up stages were both significantly higher than the pretest score (P <0.001). Also, the follow-up mean rumination score of the control group was significantly higher than that posttest (P=0.02). Comparison of the two groups where adjusted for baseline, showed a significant difference between the two groups in terms of mean rumination score at the posttest stage (P=0.01) and at the follow-up stage (P<0.001).Conclusion: Subjects in the intervention group were more successful in avoiding increasing rumination than those in the control group, an ability that must be attributed to the effect of mindfulness training. Still, further research is needed to determine whether longer interventions will be able to reduce rumination.Trial registrationTrial registration number: IRCT20090522001930N2 Date: 2018-08-08