Background: Breast cancer diagnosis and treatment can negatively impact the quality of life and lead to symptoms such as insomnia, depression, and anxiety. Internet-delivered interventions may reduce these symptoms in patients with limited access to face-to-face psychological treatments. Objectives: This study aimed to determine the effect of online mindfulness-based stress reduction (MBSR) intervention on the sleep quality of women with breast cancer in Birjand City. Methods: In this randomized controlled clinical trial, 62 eligible women with breast cancer undergoing chemotherapy in Birjand were selected using convenience sampling. They were assigned to two groups, intervention, and control, each consisting of 31 individuals, using random allocation. In addition to standard treatments, the intervention group received online MBSR training consisting of four two-hour sessions over four consecutive weeks. The control group only received standard cancer treatments. The data collection tool included a demographic form and a standard Pittsburgh Sleep Quality Questionnaire completed by individuals before, immediately, and 2 months after the intervention. Data were analyzed using SPSS 16 statistical software. The following tests were conducted at a significance level of P < 0.05: Chi-square, Fisher's exact, Independent t, repeated-measures ANOVA, Bonferroni, Mann-Whitney U, and Friedman. Results: The mean sleep quality scores decreased immediately and 2 months after the intervention compared to before in the intervention group, while it increased in the control group (P < 0.001). Also, the independent t-test showed that the mean sleep quality scores decreased in the intervention group from before to immediately after the intervention, as well as from before to 2 months later (P > 0.001). In contrast, the control group patients experienced increased sleep quality scores (P > 0.001). Conclusions: Mindfulness-based stress reduction significantly improved sleep quality in the intervention group, as demonstrated by decreased scores immediately and 2 months after the intervention. On the other hand, the control group experienced an increase in sleep quality scores. These findings highlight the effectiveness of MBSR in enhancing sleep outcomes.