Harsh or extreme environmental conditions largely determine the vegetative and reproductive development of plants. In the case of cultivated plants, their growth and yield are clearly diminished if they are exposed to severe conditions such as drought, waterlogging, extreme heat or cold, UV radiation, or toxic substances in the soil such as salts, heavy metals and pesticides. Melatonin has been studied for decades as a molecule capable of reducing the negative effects of abiotic stressors by increasing tolerance to these adverse growth conditions. This work presents a review of the most outstanding studies with various plant species in each of the above-mentioned stress situations, including proteomic and post-translational studies. Melatonin mediates plant responses to abiotic stress, generally inducing an antioxidative response, and also regulating a complex gene response adapted to individual stressors. Plants are able to increase their endogenous melatonin levels through the application of exogenous melatonin or through the inductive mechanism of endogenous melatonin biosynthesis. In such ways, plants are able to cope with the stressful situation at hand, accommodating their metabolism, morphology and physiology in order to increase overall survival and induce greater tolerance to stress. The agronomic implications of the use of melatonin are discussed.