BACKGROUND: Entering old age is associated with various physical and psychological disabilities. Therefore, the aim of this study is to determine the effect of mindfulness-based stress reduction program on emotion regulation and sleep problems in depressed elderly. METHODS: This study was a clinical trial conducted on 60 elderly individuals with depression using purposive sampling. These elderly were referred by geriatricians and were included in the study based on the inclusion criteria. The participants were randomly assigned to two groups: the Mindfulness-Based Stress Reduction (MBSR) group and the control group. Both groups completed the Geriatric Depression Scale (GDS), the Gratz and Roemer Emotion Regulation Questionnaire, and the Pittsburgh Sleep Quality Index before and after the intervention. The MBSR sessions were held for the experimental group in 8 sessions of 90 min each, once a week. Finally, all the data were analyzed using SPSS software version 26 through descriptive and analytical statistics such as mean and standard deviation, t-tests and mixed analysis of covariance (ANCOVA) with repeated measures. RESULTS: The results showed that the MBSR intervention led to a significant reduction in depression symptoms (p < 0.001) and improvement in emotion regulation and sleep quality (p < 0.001) among the elderly participants with depression in the intervention group. DISCUSSION: The results of this study showed that MBSR can be effective in reducing depression levels, improving emotion regulation, and sleep quality among depressed elderly individuals compared to the control group. Caregivers and psychotherapists of nursing homes can use care programs such as MBSR program to improve the physical and mental condition of the elderly. TRIAL REGISTRATION: First Registration: 13/01/2022, Registration Number: IRCT20211118053099N1, Access: https://www.irct.ir/trial/61207 .