The coronavirus disease-2019 (COVID-19) epidemic is associated with impaired sleep quality in nurses for several reasons. The present study aimed to determine the effect of an online mindfulness-based stress reduction (MBSR) program on improving the sleep quality of nurses working in the COVID-19 care units. In this randomized controlled clinical trial study, all nurses in the 2 COVID-19 patient care units were randomly assigned to the control and intervention groups. The MBSR program was implemented online for 7 weeks for the intervention group by a trainer. The Pittsburgh Sleep Quality Index (PSQI) was completed online by the participants in both groups before and after the intervention. The results of the data analysis indicated that the intervention improved the scores of subjective sleep quality, sleep latency, and sleep efficiency in the intervention group. In the control group, there was a significant increase in the scores of subjective sleep quality, daily performance, and the total index score in the posttest. Besides, there was a significant difference between the 2 groups in only 2 components of sleep latency and subjective sleep quality. The MBSR program can be an effective intervention to improve the sleep quality of nurses working in COVID-19 intensive care units who are at risk of sleep quality disorders in stressful situations.