Since its discovery in plants, extensive research has been conducted on melatonin which was formerly known to be a hormone existed exclusively in animals. Studies have shown that melatonin was indeed present in every plant species tested and that it had similar roles as in animals. Main roles of phytomelatonin are to regulate circadian rhythm as well as growth and development and to enhance tolerance to various environmental stress factors. Evidence indicates that plants living under stressful environments have higher melatonin contents compared to those live under normal conditions and that exogenous application of melatonin can also improve stress tolerance of plants that do not produce enough melatonin endogenously. Additionally, new lines in some crop species that possess excessive melatonin production traits have recently been developed through genetic engineering. This review mainly focuses on the discovery of melatonin in plants, its distribution among plant species and on the regulatory effects of melatonin when plants confront with harsh environmental conditions.