Cancer remains one of the most devastating diseases in the world. Breast cancer is considered the second most widely common among all types of cancers. Melatonin (N-acetyl-5-methoxytryptamine) is a natural hormone secreted by the pineal gland, acting on regulation of several physiological processes such as circadian clocks, acting as antioxidant, anti-inflamatory, and apoptosis regulation. During the recent decades the level of melatonin in patients has been linked to cancer progression and inhibition. In circadian cancer biology, the question how melatonin level plays a role in regulation of cancer progression and metastasis, including breast cancer is a major question. In this review, the molecular actions of melatonin and its prospective role in regulating breast cancer progression and metastasis have been discussed.