Background <p>Informal caregivers play a crucial supportive role throughout the course of illnesses, particularly in patients with cancer. Following a cancer diagnosis, informal caregivers often experience substantial psychological, physical, and social stress, which may lead to conditions such as anxiety and sleep disturbances. Therefore, implementing interventions aimed at reducing anxiety and improving sleep quality is essential. The present study aims to assess the impact of mindfulness-based stress reduction (MBSR) training on anxiety and sleep quality among informal caregivers of patients with cancer.</p> Methods <p>This randomized controlled trial was conducted in 2024 on 66 informal caregivers of cancer patients in Bushehr, Iran. Participants were selected using convenience sampling method and then randomly assigned to either the intervention or control group. The intervention group received eight sessions of MBSR training, each lasting 120&#xa0;min, which included mindfulness meditation, body scan, and mindful breathing exercises. The control group received no intervention. Data were collected using a demographic form, Spielberger State-Trait Anxiety Inventory (STAI), and Pittsburgh Sleep Quality Index (PSQI). Data were analyzed using SPSS 26.0 through descriptive statistics, independent t-test, paired t-test, and ANCOVA at a significance level of 0.05.</p> Results <p>The results indicated a statistically significant difference between the two groups regarding both anxiety and sleep quality after the intervention (anxiety: <i>P</i> &lt; 0.001, ηp² = 0.22; sleep quality: <i>P</i> = 0.004, ηp² = 0.12). In the intervention group, the mean anxiety score decreased from 105.94 ± 11.73 (pretest) to 80.39 ± 13.68 (one-month follow-up), and the mean sleep quality score (PSQI) improved from 11.26 ± 2.65 to 9.94 ± 2.09. In contrast, the control group showed no significant improvement.</p> Conclusion <p>The results of this study suggest that mindfulness-based interventions may help reduce anxiety and improve sleep quality in female informal caregivers of cancer patients. This intervention shows promise as a potentially effective approach for promoting caregivers’ mental health. However, the findings are only applicable to women and cannot be generalized to male caregivers, who may experience different social and emotional burdens. In addition, the use of a passive control group and the dual role of the instructor (intervention delivery and homework monitoring) represent important limitations. Furthermore, due to the small sample size, use of convenience sampling, and partial blinding, the results should be interpreted with caution. Future research should include both genders, use active control conditions, and conduct larger long-term randomized trials to confirm these outcomes and assess their generalizability across different settings.</p> Trial registration <p>IRCT, IRCT20240310061244N1, registered prospectively on 2024-04-30.</p>